Lag in Sims 4 increasing over time

@biscuitist  I don't think there's a good answer to how large your save should be.  I would be alarmed if the size increased rapidly, or if a new save (even with super sims and an equally-super mansion) was above... I'm not sure, maybe 40 or 50 MB?  It's hard to say with how much the game has changed over the years.

What I would do in your position is try the sims and the house(s) separately, maybe just letting the game clock run for a while as you do something else, and see what you get both for the initial size and the size after a sim-week or so.  If you want more data, you could also download other super sims and mansions from the Gallery and see how they behave.  I'd avoid older uploads since, again, the game has changed a lot, but anything post-For Rent would probably be fine.  Probably.

I wish I could give you a better answer, but this isn't really a science or even an art so much as voodoo.  On the plus side though, even in the unlikely event that none of your work is usable going forward, all you stand to lose is some time.  And it's worth keeping the sims or even the save on hand to test with future patches, in case something gets fixed in the future.