Having your massive battleships be more problematic logistics wise then several smaller screens is a good thing, not a bad thing.
I understand the concept, but I do not agree with your proposed method. We're in space here, we're not putting these ships in a drydock. Also, this is maintenance, not armor repair (which is done in a shipyard already).
This is a "future" scenario, in which you can suppose EXTREME modularity in ship building. I would assume that the jump drive, say, of a battleship and a destroyer are actually pretty similar, just the battleship one has a lot more "small jump modules", thus being bigger. So I do not see a reason to add a "maximum ship size that can be maintained".
That would only revert us back to the fact that you need like 150 or so maintentance facilities in order to maintain a battleship, and so you have just one or two planets who can do that, while everywhere else you cannot do any maintenance at all.
What could be done instead, IF Steve wants to keep some difficulty in maintenance of larger ships, is a non linear increase in maintenance time when the ship is larger than the sum of the maintenance facilities present at a certain location.
Say, if a ship is 30000 tons, and the place only has 20 1000 tons maintenace facilities, the effective rate of maintenance is not 20000 but 15000 or 10000. An appropriate formula would have to be proposed, one that is sensible and encourage you to use more facilities, while not being excessively punishing in case you are "below cap". Probably with a cutoff point beyond which it does not get any slower.
I still prefer linear and no limits, mind you. Just stating that an increase in maintenance time would be appropriate and preferrable in case a limitation of some sort has to be put in place, compared to a "hard block" which completely makes maintenance impossible if you do not have x facilities.