Consider what exactly you plan on starting, or growing… is it a few cold months of the year or growing throughout the year? Is part of it as a place to relax or strictly for seed starting? Veggies. Flowers? Overwintering house plants? With these questions answered then you can decide on heat source, watering, electricity… I have 2 now and the 3rd going up in spring.. all 3 are different too.
Mine is 6 feet wide, not many options for floorplans. 10' is a good width. If your building with 2x lumber, make it multiples of normal lengths: 8', 10', 12'. Rafters and studs the common width of the panels, even if you use film/sheet to start.
It comes down to these factors: how much time can you dedicate to it, how automated are you going, are you retiring soon and will that change the time you will have, and I don't care how automated you go, you still have transplanting, pruning etc.
Yes, almost like chicken math. I started with a 10 x 10 , next I added a 16 x 20 and this spring I'm adding another 16 x 20. I found I really enjoy gardening inside more than outside. I control the water, wind, and light. Less weeds and pest to deal with. I also have 17 raised beds, a tomato trellis, melon patch and potato patch outisde of the greenhouse.
Yes! They always seem much bigger until you get benches/tables/tools/etc in there and then all of a sudden you're like "Where the hell are the actual plants going to go?" lol.