You should answer the question: What will I grow? I increased the height of my greenhouse from 9' to 10' by adding a 1' high foundation wall. I did this because: 1) I grow some plants and small trees and want to give them as much headroom as possible; 2) I have some taller plants that I overwinter in the greenhouse; 3) there's more room for hanging baskets without bumping my head; and 4) taller ceilings just feel better to me because my greenhouse seems airier and more open with a higher peak. The downside is that you increase your winter heating costs slightly with a taller greenhouse.

The only negative I can think of is increased heating costs. It's extra space to heat in winter. The positive is more head room for fans and hanging baskets.

Thanks for the comments.
Heating is not my major concern as my winters are fairly mild compaired to you poor people further north. What is my concern is the heat. I am looking to make a COOLER GH for the 5 months here with really hot weather. Right now it is 91, but the high today so far was 96.4.
1eyedJack and the Dawg

In warmer regions, taller greenhouses would be desirable because the heat collects at a higher strata and away from the plants below.

Obviously, fans and air circulation devices agitate the air inside and breaks up this stratification but you cannot change the tendency of hot air to rise above cooler air.

Consider an 8-foot tall GH, now imagine that the "warm air layer" towards the top of the GH is 2 feet thick.
This only leaves a "cool air layer" of 6 feet from the ground up and the air within this layer gets warmer as you go up.

Now lets consider a 15-foot tall GH. Even if we had a "warm air layer" on top at 4 feet thick, we still have a "cool air layer" of 11 feet from the ground up.

Now these figures are not precise calculations. This is just to present the idea of how the taller greenhouses provide the benefit of moving the warm air further up and away from the growing section of the GH.

Taller greenhouses of course, will have more surface area exposed to the sun. That is why my taller GH example above had a 4-foot thick warm air layer compared to the 2-foot layer of the previous GH example.

Taller is not the only solution though. A wider or longer greenhouse will also provide this benefit. By increasing the footprint of the GH, you are increasing the area where the hot air can go to, so it spreads out thinner.
**BUT this requires proper balancing of how much "warm air" space you gain versus the additional surface area you expose to the sun. You would want this "attic" to be capable of holding more hot air than the solar load produces.

Bigger greenhouses will also have higher heating costs if you provide artificial heating.

I would like to add:

That is why sometimes it is much better to set up a shadehouse/lath house instead of a GH for regions that have warm weather year round.
The open structure of the shadehouse provides protection from the sun, while eliminating the possibility of hot air collecting inside the structure.

In my case, I thought I could get away with just a shadehouse. Last year's frost proved otherwise lol.

If you use a standard door height be sure to have enough extra height ABOVE the door to install a fan for hot weather. I have learned that a fan above the door will keep the hot air from accumulating at the top and keep cooler air circulating.

I should have 25 inche PLUS above the door so that will not be a problem. I have not yet decided to mount two doors or only oney one. Probably only one with the fan on the other end. I have found some really cheap automatic foundation vents that open at 70 and close at 40 and I believe I will use them, probably fout or six. I may make one other opening for a SWAMP COOLER or might just let it deal with the air inside.
Thanks for the info.
1eyedJack and the Dawg

Even a high ceiling isn't going to cool it much unless you have a good exhaust system and shade cloth. My girlfriend Sharon (now deceased) had a high GH and it was every bit as hot as my Rion. But like myself, she didn't keep plants in it over the summer.

Build it tall, and you can mount a pair of 52" outdoor ceiling fans in it and move a lot more air for less cost than with any other type of fan. And, do it near silently too.

But for ceiling fans to be most effective, they should be mounted directly below roof vents.