A Dutch Commonwealth would have been possible if the Dutch East Indies never fell to the Japanese. Best POD for this would have been a victory at the Battle of the Java Sea, perhaps if ABDA had better coordination that day between the naval forces of those four nations. Even better if you had an earlier POD where the Dutch flood the Holland Water Line in the Battle of the Netherlands and either sue for peace with Germany or somehow survive a year long siege concurrent to the fighting in the Indies.
Without the Japanese destroying the local colonial administration, it seems likely that Indonesia would receive independence by the 1950s, possibly with Queen Wilhelmina remaining head of state for at least a while in a way similar to the OTL British Empire.
Another possibility is an agreement being reached where Malaysia and Indonesia achieve independence together as unified state, such a power would become as strong as India eventually becomes in OTL.
South Africa is another candidate for joining this Union, in a way similar to how Canada is simultaneously members of the Commonwealth of Nations and Francophonie. If super Indonesia became a precedent in this world, the simultaneous membership in both the British and Dutch commonwealth might have actual teeth in real political terms instead of just being a nod to a shared language.
It would depend on how the Dutch approach a South Africa that was fast on the tract to going full apartheid in 1948. Perhaps joining the Gemenebest would push South Africa towards liberalisation, as the widespread anti-British sentiment could manifest itself as seeking closer ties to Europe instead of giving the middle finger to the rest of the world by becoming a Afrikaan nationalist pariah state. The strongest possible Gemenebest South Africa might be if some crisis over racial integration caused South Africa to be partitioned into three states. Cape Province gains independence from Pretoria, the other provinces of Natal, Transvaal and the Free State become one country, perhaps called the "Volkstaat" or something, and Southwest Africa gets independence as Nambia with black majority rule.
Cape Province ironically enough has the highest population of Afrikaan speakers, of all races, compared to the former Boer republics.
This is because the Voortrekers weren't interested in assimilating the native population as much as they wanted to lord over them, compared to Cape Province that until around Cecil Rhodes's time had
a well established tradition of racial equality in politics. Perhaps there would be a revival of this way of thinking in and independent, pro-European Cape Province, and the widespread prevalence of a Dutch dialect in Cape Province amongst all races, not just the Boer ruling class, would make such ties to the Netherlands more viable.
A Xhosa nation state is fourth possible state to come out of the partition, being allowed to leave by Cape Town. Additionally the Tswana areas in the North accede to Botswana.
What ever happens, it seems unlikely that South Africa or a rump state thereof would want to become subjects of the Dutch Queen after kicking out Lilibet, but it
might happen.
At this point if the Netherlands has Indonesia and South Africa in it's Gemenebest it might butterfly into Suriname becoming like French Guiana is today, an integral part of the Netherlands and more prosperous than OTL.
So that's basically the largest possible Dutch Commonwealth. Unless you wanted to go
really crazy and add Ceylon. Ceylon had a population of "Burgher" people descendant from Dutch Colonizers from the 1600s on. In 1946, they represented 0.62% of the population. Not much at all, but it was higher in the cities and these people had more influence than their numbers suggest. If Ceylon had some nasty political crisis in around the 50s between the Tamils and the Sinhalese, the Burghers and the Dutch language might end up playing into whatever power sharing scheme ends up getting worked out. Dutch might end up becoming the lingua franca in Ceylon for no other reason than it doesn't favour either side. Picking Dutch over say English would be more likely if the Netherlands retained ties to Indonesia and remained a power in that area of the world.
So, a map of Greater Netherlands, including annexed Flanders, would look like this: