You've had a couple goes on prone boards right? Am I delusional in thinking I can learn on a prone board? (I learned to prone foil on a 4'8" in waves, no boat/ski to tow me and learned fine, so I must be pretty stubborn)
My current plan is a 5m f-one. I figure a slightly bigger wing to counter for the small board and foil. Plus I have old kite harnesses around so I can try out a harness line too.
I don't know your ability but I can say it was pretty tough getting it going. The young guys seem to be more consistent getting it out of the water. But when the wind goes light they're screwed. That initial lift of getting the board to the surface takes alot of work. After that it gets easier.
The other issue you may have, the 5 meter wing is pretty big. And to muscle that thing into a position to get you going from half of your body underwater will be a challenge.
You mentioned small board and foil...how small foil? I'm switching around my foil wings quite abit based upon wind conditions. 2 days ago I went chasing wind to the other side of Maui, it's been light here for about a week and wanted to wing. I have a handheld wind meter I use on the beach and it was down in the 10-12knt range. I normally use my GL180 in good wind conditions (15-25knts), I used my old M200 and was constantly either pumping the wing, or foil, or both with a few little moments of relaxing.
I'm going to say, from my experience, is that you want to be sitting on the surface to get winging. I usually look at the tradeoff of the pleasure/pain ratio in making a decision like this. If it ends up being more frustrating than fun, I ditch it. And using a small prone board and winging is not in the cards for me. I've tried it, was alot of work to get it going, and all the time I was trying to get it going, all my other wing buddies were off having fun winging their SUP boards.
Not to say you have to use a SUP, but a small board with enough volume to allow you to stay on the surface while you negotiate getting to your knees then stand. For me that's going to be something with volume of at least 70'ish liters.
And I've had a few conversations with DK (Dave Kalama) about this very topic, when I was all amped to wing a small prone board. And his answer was, what's the benefit of using such a small board? It didn't make sense to him. And my only answer was you could do airs alittle easier. Other than that, there was no clear benefit. Winging, in surf, you're having to hold this big object behind you so you're not doing much manuvers that would warrant a tiny board. You can get away with a small SUP.
So that's my .02 on the subject. Let me know what you decide on doing and how it goes.
On a side note, my buddy is selling me his 5m F-One this month so I should have some feedback on a bigger wing.