So easy to legally bypass size limitations.
Example: limit - 250 MBTs.
Buy 1000 and keep 750 of them in storage. Talking shrink wraps and all.
Train enough crews to man double that, and have the crews rotate on tanks. Those on downtime do simulators and drive on empty hulls made for drivers training.
Storage limitations as well? No biggie. Convince the US to vastly expand its European WRSA and get them closer to Ukraine. Funnel funds saved from reduced defense budget to buy into those WRSA and flex by several hundred percent in size in the course of one week. Location means WRSA are immune.
Basically limiting Ukraine's army size will hardly have any effect on Russia's balance on it.
As long as Europeans are awake to the threat and investing in defense, Ukraine can be almost endlessly restocked.
If Ukraine and NATO play it right, a tactical victory in Ukraine could be a strategic one.
Once Russia withdraws, Ukraine could be admitted to NATO and then it's immunity from Russia.
Size limitations might also include service member numbers limitations (again like Germany post-WWI). And again this is just my speculation, so feel free to take it with a grain of salt. And a pile of US weapons in Germany or France would take time to reach Ukraine. Especially when we're talking about equipping entire units out of the box. To top it off, Ukraine hasn't exactly done well in training or structuring their force before this war. Between rampant corruption and just general inefficiency, there's no reason to think they can keep tank crews well trained on shared MBTs when they hard a hard time doing it on not-shared MBTs.
To be clear, the scenario you describe is totally possible, and if we were talking about a western military, even probable. But it's Ukraine, so who knows. It might work out that way. It might not. And it might be that Russia partitions Ukraine, when they discover that the west isn't willing to make deal that they're happy with. What starts out as a negotiating position (for example "we will stop occupying Kiev, and south-eastern Ukraine in exchange for lifting of certain sanctions, and a neutral status for all of Ukraine) could turn into a permanent stalemate if/when a "rump Ukraine" government in L'vov, with western backing, refuses the deal completely leaving Russia sitting with territory it doesn't necessarily want, sanctions that are downright crippling, no way to back down without losing, increasing costs to just sitting there, and no way to move forward without an even bigger escalation. And then there won't be any restrictions on the size of Ukraine's military, other than the resources the west is willing to pour into it, and the physical manpower available in the remaining regions under Ukrainian control.
I honestly don't know. As I said before this invasion, I don't see what Russia has to gain. I think it was a bad move, with no real way out and it's not clear how Russia intends to move forward. I don't see any possible gain that can offset the damage of sanctions and political fallout from this, even if we assume that some of the worst sanctions get lifted shortly after the war ends. And that's a pretty big assumption (going into it are considerations that Russia will leave Ukraine after some sort of settlement, that the West, Russia, and Ukraine, will all agree to a settlement, and that this settlement will be enough to lift some of the worst sanctions).