Largest possible US?
OK
With a POD in 1963
Cuban Missile Crisis (or equivalent) goes hot
The US his hit hard, but much of Europe is a wasteland, and the Soviets are crushed.
By luck, the ICBM targeted at Washington DC misses.
US Congress looks at WWI, WWII and WWIII, and says 'never again', and the Johnson Doctrine says "no power will ever rival the US again".
Congress also passes legislation saying 'USA First', that the US will only provide reconstruction aid to areas of the US which have been damaged.
Denmark is the first country to apply for Statehood - so they can get help rebuilding. This is followed by the Netherlands and Belgium. Yes, they have to give up their royals, but that's survivable. The US worries about their 'foreignness', but accepts them as protectorate/territories initially so they can get aid, with future status to be determined later.
Meanwhile, London was hit during the war, taking out Parliament and essentially the whole royal family, leading to chaos as there is no central (and little other) authority. Realizing the problems the mess they're in, Scotland follows the lead of those other small European nations, and applies for Statehood. They are followed by Greater Yorkshire.
With the Royal family gone, and 'Britain' in chaos, and the nativist mood in the US, Canada realizes that the writing is on the wall, and applies to join. They negotiate a deal where the various province keep their names and governmental structure, just making the Lieutenant Governors explicitly appointed by the local legislatures (de jure now, not just de facto). While this confuses the US, it technically meets the "Republican form of government" laid out in the Constitution, and 8 new States are created. (Quebec decides to hold out for the moment, and PEI is just too small to be a state). Canadians, being seen as 'odd Americans' anyway, are an easy sell.
Once those 8 states are accepted, Scotland and Yorkshire are no brainers. And the rest of the UK is accepted as a territory.
Now, Denmark and the Netherlands and Belgium say 'hey, we were first, what about us?'
And formal accession criteria are established.
Any new prospective state has to have a functioning democracy, at least 80% literacy, a certain degree of economic development, and agree that all Federal communications (including debates in Congress), are to be in English.
With that proviso, the 3 European former countries swallow hard, and agree.
Australia joins, each state as a state, and New Zealand as one.
Sicily applies next, and then the rest of England (as a couple more states).
France holds out for some considerable time, as French chauvinism is huge. Germany, well, there's not much left of Germany, but they become a US territory.
US tariff barriers rise, and export restrictions increase (no country outside the US sphere is going to get modern military equipment, for instance, and even modern industrial tools are doubtful - unless owned and controlled by US companies).
Ireland joins.
Argentina joins - as two states; followed by Chile.
By the year 2000, most of Latin America has accepted US dominion, at least, and countries try to meet the necessary targets for statehood.
Most of Europe, too, is US controlled, although much of central Europe is in such rough shape it can't meet the minimum requirements.
Over the next decades, Korea, Japan and Taiwan join (as various numbers of states), and the Pax Americana (although 'America' has been dropped from the US's name by now) holds.
Even France finally gives in.
So, by 2050 the US (longer the USA) covers (states, territories and protectorates) all of North and South America, Europe to the Urals, and parts of Asia. Soon thereafter, an official Act passes Congress, stating that the US covers the entirety of Earth, and indeed, wherever humankind exists.