If you have a style already in mind, I would say for two people start out with 8 to 10 feet wide, and 20 feet long in whatever configuration works, and expand as you go.
I built five 10x19 foot quonset hut style greenhouses 25 years ago, and they have evolved to two of them holding fruit trees with just chicken wire covering and some fabric in the winter, and the other three has vegetables. The reason I say 19 feet long instead of 20 is that the greenhouse plastic comes in 20-foot lengths, and you need 6 inches on each end to roll around the ends to seal it. (As an example, the ends will be flapping open if you put a 20 foot piece of plastic on exactly a 20 foot frame.)
The trees are frost sensitive, banana, avocado and tangerine, lemon.
A quonset hut is 20-foot rebar, covered in 1/2" PVC pipe, bent in an arch, ends 10 feet apart. Ends inserted into the ground, but the ground doesn't freeze here, so that extra depth beyond frozen soil would be part of the calculation. It is about 71/2 feet high, arches are about 3 to 4 feet apart. There are wooden supports going down the middle because the center of the rebar, over time, will start to sink, especially with the weight of plastic or panels or shade cloth on it.
This style is really good in bad winds. We had what they call a microblast, approx. 80-90 MPH winds for 1-2 steady minutes, not a gust, everything shook, but the greenhouse had very little damage. It can be added onto if you want. It's DIY, so it's not as expensive as a kit. I put a chicken wire wall between two 19-foot sections so if a packrat gets into one section, it probably can't get into the other section, and I can get rid of it before too much damage is done. The door is wide enough to accommodate a wheelbarrow.
Not sure why people want really high greenhouses. I want the heat down around the plants, not up at the 8 or 10 foot height. I also don't want to have to get on a ladder to repair them. I don't want them up in the wind. Greenhouse plastic comes in specific widths, and a 20-foot width of plastic is what covers an 8-foot wide arched greenhouse. I put 2-foot wide, 8-foot long patio panels around the bottom and the plastic comes down over the patio panels. It keeps the animals from seeing all the green in there and trying to get to it. Deer will try if there's a drought and they are low on food. Foxes, and rabbits, too, they love tomatoes and fruit.