Scale length within limits. I can play on any 25.5" or 24.75" scale guitar comfortably. I tried 24" before, it's OK, but the frets can get too close to each other fast as you go up the neck. Anything shorter is a no go for me. I've played 27" and 30" baritone necks before too and they were cool.
As far as radiuses, or is it radii, anything between 16" and 9.5" is fine, after a couple minutes I'm comfortable and don't notice. Compound radius anywhere between 9.5" to 16" is also fine. I have tried 7.25" Fender necks before and my recollection is that they were fine, but I think I'll set the lower limit at 9.5".