there is no magic bullet. Try different lengths to see what you like. Chances are if you only ever ride 170 or 175 you won't notice much difference. 165 and 185 you will notice something. I have ridden 165 to 185... I like 180 but also road 175 for most of my life without ever thinking about it so I am habituated and like the extra leverage I get with 180 but find 185 is a little too much to turn over on the road bike for long distances. Again that may be just me not being well adapted to that length and if I road 185 all the time I might find that I prefer it to 180... but 185s are rare and expensive and 180s are relatively plentiful by comparison. I would say if you are average height you should go for 165 or 170