I was thinking of getting a fatbike . quite like the On-One fatty but not sure about size .
I`m 5ft 9in with 29″ leg
16″ or 18″ frame ? I want to use it for mainly trail riding .

thanks

16″

5’8″ with 31 leg and I’ve a 16″ one.

Perfect as a play fat bike
If you want more of a touring feel go for the 18″

On one fatty has the same stand over height …if that makes any difference ………oh and I’m 5.10 30″ leg and ride an 18 . 8)

18 incher for me at 5 ft 9 in with 30 leg and feels right

sounds like you are on the border so a try on one would be a good idea

OP – per On-One’s sizing guide, you’re a 16″ (Small).

Sizing Guide
Rider Height Inside Leg Suggested Frame Size Stem Length
5?6? to 5?10? 28”-31” 16” 40-60mm
5?9? to 6?1? 30”-34” 18” 40-60mm
6?10″ to 6?3? 33”-36” 20” 40-60mm
Sizing Notes

All frame size recommendations are approximate. A good fit will also depend on the correct choice of stem length, correct bike set-up for the rider and riding style.

Correct frame size choice can also depend on a rider’s torso and limb length.

thanks guys . still not totally sure ! but edging towards 16″ . I was going to run a layback post and a 50mm stem