Growth Cab

I'm a complete noob in the field, so I've got a couple of questions(probably will have much more in the near months):

What size would I need in order to grow 2 plants?

Also, I might have to buy a closet, but I was thinking iit might be better just to build a wooden frame and use some air tight material as the 'walls', what do you think?

1 sq foor per plant is the formula I "know" but I wouldn't grow anything in less than a 4 x 4 space. Better to have too much room than not enough. I built my own grow closet/tent, airtight isn't as important as light proof tho, meaning no light can get in from the outside.

Thanks for the help.

Can you recommend which material should i use for the tent?
Will it hold with all the pipes and electric cables going through it?

And yeah, I know light from outside is a no-no, they need their sound sleep. =)