Recently bought a bfe . It has a bb width 73mm and am looking to buy the raceface respond double chainset . There are 3 to choose from . 165mm , 170mm , 175mm . What one is it .? Cheers

Buy all three, try them and report back.

I always use 175’s and they seem to work ok.

What size frame, how long are your legs, and how long is your femur? Riding style, spin or grind?

They’re different measurements.
You need to buy something that work with the width of BB shell (and correct chainline) and also buy something that has the right crank length for your legs (although Mike Burroughs is currently saying the different crank length isn’t anywhere near as important as had been previously understood).

Mike Burrows last time I heard was spending a lot of time shortening cranks, some at crazy short lengths as in the region of 120mm!! The guy is certainly a genius most of the time, just be interested to see if he is on this issue.

I’ve got an assortment of 170s and 175s, they just feel the same tbh. I could feel a wee bit of difference going down to 165 but even then it wasn’t much. Not worth worrying about IMO.