My space is 35" h, 35" w, and 23" d. Im using 0.67 gallon folgers coffee can hempy buckets, used a scrog setup and set my screens 8" above the bucket. I vegged till the tip broke through the screen and flipped, I coulda vegged a bit longer. If not scrogged I would say the plants would be around 18" high. They are about 10" high right now. I have pics in the link in my signature.
I dont have much of an idea how to guess weight when it's on the plant still but I am estimating around an ounce each. So depending on what type of overall yield you are lookin for 1 gal hempy buckets are just fine. The ones I am using for my next grow are 5qt/4.7L and this gives me about 80% more volume than my coffee cans. I would definitely agree that for your space 3 gal would be the max I would go.