I think they all have more or less the same area, so it depends more on the most practical shape. Concordantly, while some may appear to have larger areas, they actually have abot the same area as the smalles bases, just arranged differently.
What you want is a short, wide room instead of a long, narrow one. The one on the route east from Fortree City has probably the best internal arrangement, and it also exists in the area north-west of Fortree, just it's not really accessed easily.
Have a look around, and choose the one which will be most practical for the items you want to put in it. If you want 2 slides and a stand in it, you're not going to get one of those "long, winding passage" kinds of bases.
Apart from the one next to Fortree, I also recommend the one on Route 110 (on the island) which is fairl large.