I'm using the Hydrohut's Silver edition 2x4 tent. The tent is acutally 28" by 58" x78" and was perfect for the room I had available. I am able to do 6 plants comfortably in a SCROG type setting, I have 1 400wMh and 1 600w HPS running at the same time and with a few fans and ducts added, heat has never been an issue.
Make sure you stay away from any tents with the white interior. I read some of the older tents using the white reflective material were leaking chemicals that were killing peoples plants. Stay with the silver reflective ones...
Make sure the tent you get has plenty of pre-fabricated holes for venhilation, air intakes and outs, cords and accessories.
A nice feature with mine is that is has a 2nd floor to it, one that is removable between grows for easy cleaning and to help with any spills....
I also like how the tent I have has screened vents that can be opened if needed just by undoing the velcro.
make sure if you are ordering one, check hydro stores and then check amazon...I noticed that amazon has the same tents but for much cheeper than the hydro stores websites....but also read people comments...some of the cheaper tents have crappy zippers, light leaks and weak frames...
When planning out my tent sizes...I actually measured out the area that it would fit in...and taped the area off with masking tape to help visualize how it would fit in my room...this really helped me determine if the size was right...
Make sure you check the dimensions of the tent...for example...mine is advertised as a 2'x4' tent but it is closer to 2 1/2 foot by 5 foot...if I wasn't aware of this before the purchase...I might have screwed myself...
Check craigslist for used tents....I am here in Cali and almost see a new tent every day listed for sale...many going for half price....I ended up buying my tent, with 400w hps light and ballast, and 6" 400cfm fan of a guy on cragislist for 250$....a 1/4 of the cost if I would have bought it new...and the tent was in a sealed box...unused...!!!
Hope all this helps...I shopped for a good month before I purchased...and ended up saving a good 500$....