I got an old enlarger that takes up to 6x9 for free, it's actually made in Sweden! The thing mounts on the case using it as a baseboard. And as you can see you've got plenty of room for paper. This case is hand made and damn heavy, but it's a neat setup.
The idea of a table with wheels is not bad either, since you can make it fit over the toilet. Either way, I think that you need to forget about the included baseboards and make one yourself. Then 4x5 would be viable as well.
I've got an Durst M301 with the color head, it's so damn small! It's capable of making 11x14, if you don't use the masking frame. It's small enough to fit in a standard bookshelf and you can lift it with one hand without a problem. I intend to keep it even if I get a 4x5 enlarger (I'll probably go with a custom made baseboard/table then) just because it's so damn small and easy to work with. The M600 isn't much larger, but it seems that you can't get a color head for it. The 605 on the other hand... A tad bigger, but with a small baseboard it should work.