I need to get some new cranks, does anyone know how you work out the best length to use?

175 is the standard length for MTB for average size people. unless you are very tall or very short, i would suggest 175 will be fine.

hoe tall are you?
and whats your inside leg (oo-er)?

Try the sizing system on the right hand side of the page.

Try the sizing system on the right hand side of the <a href=http://www.wrenchscience.com/WS1/default.asp">page.