I am 35 and want to race again for the first time in 20 years. I am rebuilding my 24". Should I get 180 or 175 crank arms?

try one of each...
seriously it depends on your height, are you riding 20 or 24 inch, gearing and finally and most important, what feels right.
I'm six ft. so according to most I should run 180'S but I've always prefered 175's.
Good luck with the racing!

Wayne

oh yeah, I am 5'9". I am ordering parts and I am on a budget. The consensus seems to be 175, so I think that is what I will order and see how it goes. It has been a long time since I raced so I guess it is hard for me to get out of the cruising around town mode of long cranks and tall gears.