Ok, let's go one step at a time...
Step One: Do a mock-up. On a table or something similar, lay out your models (you'll want your models about 70-90% complete), figures, buildings, and infratrure in the positions that you want. Move things around, going for the best look, then measure what size base you're gonna need. Since you say you have a 1 x 2 foot plaque, it's behoove you to lay it out on that.. In the future though, you should make your bases fit your diorama, rather than the diorama fit the base, unless space is a real issue for you.
You have a lot of action going in a small space, so once you get it mocked up, you may see that it doesn't work and you'll have to rethink it... Happens all the time, BTW...
If you can swing it, buy a copy of How to Build Dioramas by Shepard Paine... It's the Bible for Diorama builders...
Also, some of Shep's tips for building dios that were published for his Monogram dioramas are HERE: http://sheperdpaine.atspace.com/flakpanzer.htm They're worth printing out, BTW, since they're highly coveted by modelers..
These diorama tips that came in the Monogram kits back in the 70s are what got me and many, many other modelers into the diorama business..