I am getting ready to build my first greenhouse. I am a researcher by blood, but not by trade.
When I put in our first pond I kept reading, "the bigger the better because you will want a bigger one if not big enough" . Though I found this statement to be true in a short time, the area where our first pond sits could only house a small size pond. Two more larger size ponds later, I think I am satisfied. I also read, "do not put your pond under a tree", for obvious reasons. That was disheartening since I have a small back yard full of mature trees. Then I found one source that said only in Texas can you put your pond under a tree. So we did! Guess what we deal with every fall? hahahahaa
OK. Back to greenhouses. I think determining size is the same for greenhouses. Get the largest you can since plants 1) do not stay small and, 2) you will underestimate your greenhouse needs almost always. Learning from my ponds, I got the biggest one that could fit in our yard. I read over and over again about overflow of plants into other sheltered places outside of the greenhouse, shelving not large enough, needing more places for hanging plants etc. Already I am thinking the largest unbuilt, GH for my yard is not large enough and have contemplated a large hoop house over the entire back half of the yard. Wouldn't the HOA have fun with that one?!
But upon further research I have found, that gardeners are a greedy sort, about space, anyway. If there is space, they will plant it in, compost in it, build on it, use it. ALL of it! You could have the largest greenhouse in the world on the property and if you are a bonafide, dedicated gardener that greenhouse will be filled from stem to stern, top to bottom, every nook, and every cranny in good time.
Sorry to report, further research finds there is no cure for this. Some have bumped out walls and constructed greenhouse additions. Others have build a 2nd, 3rd, and 4th greenhouse. No matter. In time, it will not prove to be enough space for plants, seed trays, planting starts, and all the things gardeners do within greenhouses. Greenhouses, though we say are for the protection and growth of plants, only feed the gardener's insatiable need to keep things green.