First and foremost, you definitely want the room to be under a vacuum. It doesn't need to be a lot, but it should definitely be something. That'll work a helluva lot better than trying to seal every little leak, crevice, door, etc. Sealing is ok, but pressure differentials are better. It's a common practice for labs, clean rooms, and chem/bio defense. Should work just as well for smoke rooms
Proper exhaust sizing is key. Figure out the volume of your room in cubic feet, then multiply that by your desired air changes per hour (ACH) and finally divide by 60 to get the exhaust flow in cubic feet per minute (cfm). I think common ACH is ~5, but I'd have to double check that.
For a small room (~10x10), I'd design it without a fresh air intake and rely on leakage of your smoke room for the makeup air. One benefit here is if the surrounding area is already conditioned, you won't need to worry about heating or cooling as you'll be pulling conditioned air into your space. If you do include a fresh air intake, I suggest making sure to include a damper somewhere so you can restrict the inlet air in order to maintain your vacuum. Put the intake duct as far away from the exhaust as you can to help with air flow distribution. If you can isolate the ductwork for this room from the rest of the house, do that, it'll eliminate the ductwork as a leak path.
That ought to cover it, I'll post back if I think of anything else to add.