Size: Originally I thought to go with a 24' x 48'. You know, start small, grow later. As it turns out, it's much more efficient to go larger from the outset. Cost per square foot is considerably less. So now we're looking at a 24-26' x 96'. Double what I thought at first.
Shape: Most likely it will be a rounded peak. I'm sure there is a technical term for that, but my brain is full.
Ventilation: Here's where I'm really waffling. Mechanical (end-wall fans) or roof vents with/without roll-up side walls? There are pros and cons for each and no matter who I talk to, I get a different opinion. There just is no one right answer. Mechanical is considerably cheaper to install, but will take electricity to run, so higher utility bills. Roof vents really should also have roll-up side walls for the best ventilation, but installation is significantly higher. However, cost of operation is extremely low. I haven't quite decided, but am leaning towards mechanical. We'll see.
Wall covering: Twin-wall polycarbonate or double poly film? Twin-wall polycarbonate is guaranteed for ten years whereas the double poly film is only guaranteed for four. Also, the twin-wall is much less likely to sustain damage from tree branches, stones, hail, etc., and it looks much nicer. But when you consider the increased cost of materials and installation, the twin-wall will be double the price, it doesn't quite seem worth it. So I'll be going with the double poly film.
Heating: Hands down, it will be a ceiling-hung propane blower. No question.
As soon as I make my final decision, I'll go and see what the zoning (you're not a farm) board needs to give me a building permit.