I am 5’6″ and am between small and medium size frames. I normally go for a small frame on 26 inch wheels but should the same apply for a 29er?

I’m 5’7 and ride a 16 “scandal but need a 400 seatpost so a medium would do me as well.
I would use top tube length to decide.

5’7″ here and rode a 16″ Scandal – it was by no means too large. Also tried a Medium Salsa Mamasita and I’ll be buying a Medium Salsa El Mariachi as a result.

I had a trek excaliber last year, ordered from a local bike shop who said a 17.5″ was my size, im 5.10". It quickly became apparent this was too big for me, reach and s/over, nice bike but a big ol barge, i now have a 15.5″ trek (superfly)and what a diference, bike is playfull and chuckable like it should be.
I dont know how treks sizes compare, but i would advise to go try a few first, i was surprised at my height needing the smallest frame size.
hth.

I would use top tube length to decide.

Agree this but bear in mind it doesn’t need to be identical to your 26. A bit longer is OK I find, you can shorten the stem for the same fit, or keep it the same as your 26, go narrower bars with a longer/lower setup.

i was surprised at my height needing the smallest frame size

Height is a false indicator of frame size IMO, it’s more about inseam (AKA inside leg), and reach for the top tube.

As for chuckable, I quite like that my 29 is less chuckable, I never used that chuckability of my 26 (I realise) and I like that my 29 goes where it’s pointed. (Horses for courses.)