I am beginning to learn Sup Foil, how do I determine what length paddle to get?

One thing I will keep in mind is rail damage from too short a paddle. The blade hits my board rails a lot more. If it was longer, then most times it would be the shaft contacting the rails.
And being up on foil I think it is ok to have a paddle longer than head height. Usually 6 inches or 150mm above head height I think was the average.
Better longer than shorter because you can always cut it shorter if needed.

I'm going to go a bit the opposite and say go short, I run mine about 2 inches under head height.
I go short for a few reasons which a few are.
1: getting to your feet is easier with a short paddle, you get it in the water faster and helps with riding narrower boards.
2: Paddling for waves is heaps easier as when you are doing short fast paddle strokes you can get it in and out easier.
3: Easier on the shoulders and much less likely to do a shouder injury.
4: It just easier with a shorter paddle.

Once on foil you really should try and not use it anyway as if you are paddling while on foil it means you are to low.

Like paul.j, I'd say no more than head high.
I would also add to his reasons:

5: it helps crouching down when delivering the power on takeoff, which helps immensely both in efficiency and stability.
6: once in the air, handling the paddle with only one hand unlocks your body and make controlling the foil so much easier. The smallest the paddle, the least cumbersome it is for this.
7: hitting the foil means a broken paddle. The longer the paddle, the higher this risk

I'd say no longer than head high and a fist. I personally like at my head height. I wouldn't go any shorter when starting, harder to put back on than take some off.

Thanks gents! It's still an adjustable carbon paddle that I will "fix" to length with my favorite tools. Epoxy and carbon! I'll try shorter before I bond it, I need to work on getting lower. I'm all standing tall from my sup cruising experiences and just falling over before I even get into a wave!

Thanks gents! It's still an adjustable carbon paddle that I will "fix" to length with my favorite tools. Epoxy and carbon! I'll try shorter before I bond it, I need to work on getting lower. I'm all standing tall from my sup cruising experiences and just falling over before I even get into a wave!