it's also a question about brushpoints - i've never tried a really big map - but they can be a lot bigger for sure (but higher brushcount results in higher polycount - needs faster computer)
I have, mostly when I was experimenting. If you make it *very* simplistic, the size doesn't get very bad. I played around with up to 12x12 tiles without significant issues. The map also had a very low brush count, as I was experimenting. Having seen issues with smaller maps and size problems, I'm not so sure it was a valid test anymore, however.
Regarding the large maps, I just don't think the small ones feel right. The harbor map in the campaign for example, harbors are huge. Fences or freight could be placed to block off areas of the map.
How long do you want to play the same map? I'm guessing at size because it's been a loooong time, but I'd say the 'big farm' maps were about 6x6 tiles on the currently used scheme, and the terror missions were about the size of the 'gas station' map, maybe a bit larger and with more levels, but not more size.
If there's only 5 or 6 aliens in a map, you could spend a looong time playing hide and go seek on a very large map. As it is I could see the base maps being this large. At least that should have continuous action as every solider and UGV in the base would be involved and some rediculous volume of aliens.
(Tangent... Suddenly 'base fights' are all out wars, especially on defense... I don't know what the plan is for that implementation.)
So sure, there could be larger maps. Yet, maps like 'estate' or 'towers' plays out in a nice time frame, about an hour. If it was a big broad flat mess, like bridge (which is HUGE, load it) which is 15x12 tiles... sure. It's still around an hour, maybe a bit longer as I RF sniper the guys up in the far corner and clear the junkyard with my assault team.
I'd say, in general, keep a perspective to playtime vs. # of toys.