Hrm.... Largest possible.... I'll look at the Soviets.
You need to get early expansion, both to prevent all that Socialism-in-One-Country talk and to avoid a Russian majority early on.
You probably want to avoid Stalin coming to power, as he industrialized at best slightly faster than projections of Bukharinist policies, while otherwise ruining the country.
You absolutely need a total war in western Europe, and you need it to be well begun before direct Russian involvement in the conflict.
You want the right men (ones who see annexation as a good) running things when the SSSR goes a conquerin'.
You must not allow a large-scale invasion to be even initially successful after the Civil War. The Soviet Union recovered from Barbarossa, technically, but it never came close to the momentum it had before. Plus it's most efficient not to move all your industry to the Urals.
You can't, unfortunately, get China, so you want to arrange things to take as many reasonable chunks as possible.
The Reds do better in the Civil War because *dah*dah*something*. That means a less damaged Russia, plus let's say retention of Estonia, Latvia, and a wee bit of southeast Finland. If Poland goes Russia will be in central Europe and the rest of the continent will be less likely to fight among themselves, so let's leave it independent.
For one reason or another the Russians intervene more effectively in favor of the Persian SSR c1920 and it is incorporated. The result is a new front against the British and an eventual partition of Persia between White and Red. Mongolia and Tannu Tuva are annexed as a combined federal unit for some reason (not really important, but it gives the Soviets a lot of claims in northern China).
Without Stalin's full-bore collectivization we have less severe famines, and more opportunity to deal with them, so a tremendously larger number of Russians and Ukrainians. In Germany, the Nazis aren't in power, so we can still have a viable communist movement.
Still, have war break out between Germany and, say, France, Poland, Britain, and Italy. Soviet support as a neutral is granted in exchange for inclusion of the German Communists in coalition government and a sphere of influence. Let's say.... Lithuania, eastern Poland, Finland, and Romania. Poland is overrun (we can't have a short war) and Paris captured, but Germany isn't doing as well as in OTL. Mobile offensives heave back and forth across northern France and the Low Countries.
Meanwhile, the Russians march to Helsinki in high summer, annexing Lithuania, Finland, Sinkiang (hey, almost did OTL), and the near bits of Poland and Romania. When the war in the west begins to go downhill for the Germans, they literally sell Poland to the Soviets for a ridiculous amount of materiel and men (in the form of "unofficial volunteers"). In Persia the British and Russians stare at each other hostily.
The West eventually breaks German lines and marches into the Ruhr. In desperation, the german government hands the reins to the German communists in an attempt to force Russian intervention on their side. It's too much for the British, and they do the job themselves, bombing Baku and Sevastopol.
At this point, things get out of hand. Japan declares war on the Europeans and goes haring off into southeast Asia while still grappling unsuccessfully with China. The Russians and British clash in Persia, Iraq, and Romania while both sides march into Germany. The allies get about half of the place by the time the lines form, which, given that they are viewed as occupiers and the Russians are not, is not such a great advantage to have.
On the peripheral fronts the existence of mass Russian armored columns on a fairly short logistic train is decisive. White Persia is overrun and Mesopotamia follows it. Romania falls, Czechoslovakia and Hungary are plowed over, and the Front in the Balkans is left somewhere in central Bulgaria.
The western allies have a year or two of experience at semi-mobile warfare to the Soviets' "we didn't lose to Finland", and first in Germany and then Poland they force the Russians back. Poland, and then Czechoslovakia, Hungary, and Bulgaria are liberated rather easily, in no small part because the populace is new-conquered and very anti-Soviet. The allies press on with their momentum for a combined thrust into Russia with the idea of swiftly toppling the Communist Regime.
Then they cross the border into lands that have always been Soviet, and this develops to have been a less than ideal strategy. Western Europeans didn't have the highest opinion of the eastern peoples at this time. They don't have a great deal of patience - having fought for a few years against the Russian-funded Germans. And they have unrealistic expectations of flowers being thrown at their feet. Incidents of partisan warfare are reacted to brutally. In spite of this, shockingly, more partisans appear.
The Soviets have more (and often better) tanks, friendly ground, a very large number of unpurged commanders, and now they know how to fight a modern war.
The great spearheads sweep through Lithuania into a meatgrinder in Riga and across Belarus and the Pripet to be ground to a halt in western Russia. The real disaster strikes in western Ukraine, where the mass blitz to the Black Sea is flanked, pocketed, and forced to surrender. Most of a year is lost desperately trying to keep a toehold in Russia proper, while grinding east through Romania. At the end of it, the Russians get a major breakthrough in the Ukraine, and the collapse begins.
An armored thrust races the length of the Vistula, forcing a mass evacuation from East Prussia, Lithuania, and Latvia and the loss of a mind-boggling amount of tanks and heavy equipment. The next push rushes into Germany, where the absence of a real peace and the use of German industry to help run a war against Germany's ally results in mass support for the Russians.
The rest of the war is a slogging, brutal affair, fought across the Balkan Peninsula and Germany. In the end the west is unwilling to continue a war that includes occupying a hostile Germany for years on end. Japan is losing ground steadily, and a peace deal is made. The Soviets annex Poland, Germany, Czechoslovakia, Hungary, Bulgaria, Romania, White Persia, and Iraq permanently. After a short wait, they also pile on Japan's back and annex Manchuria and Korea, plus Chinese Mongolia while they're in the neighborhood.
The new awkward superstate feels, acts, and thinks much the same way the Soviet Union did in OTL (but without the "Omg, thank God Stalin isn't killing us"). But it's stronger in essentially every way.
Oof.