I'd also suggest a
bin cabinet for storage. When I was on 2220, we had a couple that we mounted on caster wheels that stored everything from safety gear to seven seasons of partially disassembled gearboxes. They're a bit pricy, but they're fantastic things, both for teams that have a dedicated build space and those who have to be more nomadic-- especially the latter, though.
I'm also a fan of good workbenches-- whether it's a station work table like
this or a more table-y table like
this (as a side note, I'm not recommending this specific supplier-- these are just examples of what I'm talking about-- research is key!).
As for size, I'd lay out precisely what you want in it-- even if you can't afford it all now. If you want to have a Bridgeport and a tooling cabinet in there, but can't afford it now, lay that out. If you're talking 10-15 people in the shop now and maybe 30-40 later, I'd plan for 2-4 workbenches plus whatever machinery you deem necessary.
I'd definitely second/third the idea of having a "clean" side of the shop-- keeping everyone in close proximity is huge, so if you can swing it, go for it.
Best of luck with your pursuit of a new shop space, and keep us updated! It's always cool to see what teams are capable of getting accomplished!