How ever big you think you want (need) it build it bigger.

I have no construction advice, but some additional information would likely be helpful, such as:
- Will this structure be connected to or separate from another structure, such as a house?
- Can it piggy back on any existing heating/cooling/water, or be independent?
- Underground, above ground, above another structure (like 2nd floor of a garage)

From what I've seen people do, you'll need to think about everything from flooring to where you place you outlets to sound proofing. Want a bright area or dark and cavernous.

Anything is awesome, be creative, and post pics!

Go talk to your local municipal building department first.
See what you are allowed to build and what approvals and permits you need.
Hate to see people plan stuff and then find out there is no way to get the permits.

Bumping this instead of opening a new topic. I am rebuilding my basement. I want to add electrical circuits exclusively for the machines. I'm figuring 2 machines on a circuit. I have large breaker box with plenty of open breaker slots. So my question would be what gauge wire 14 or 12? What AMP size breakers?