I am wondering what size you like and why? I am planning on 16 ft wide by 32 ft long I am using trusses from a lumber yard and long 4x4 treated post it will stand 12ft on the wall end and 16 ft in the center. I am using 5 gal buckets and sterl soil / compost. I have grown outdoors for about 25 years now and am really interested to get my first greenhouse. Although with limited space I will have to downsize the hobby but I am thinking my quality will triple!
You could also use aluminum poles bent in a hoop, or pvc. Just need to figure out how big you want bigger the better. I have a 12x20x8 and I want more.... Just use scrog and you wont lose any yield