Switching to VPS or dedicated server: How do I know how much server space I need?

I’m with bluehost running a single wordpress install with buddypress. No plans to offer websites to members.

I’d like to create several more websites with the same format as above. I’d like all these websites to be on one server, one cpanel, etc.

I think I’m pushing my limits with bluehost from a cpu standpoint on my one site.

I’m looking into “managed” vps and dedicated servers. I’d like the room to grow or at least easily upgrade with the same host

My question is:

How do I know how much space I need, which plan to go with, etc.? Or at least, how does one go about figuring that out?