Idlers shouldn't be necessary. I made the mount with the holes for the stepper motor slotted to allow tensioning the belt and different size belts and pulleys.

You can't change the lengths of timing belts - the only way is to buy a different length. You can work out the length with something like this:

http://www.sdp-si.com/cd/default.htm

Also:

http://bearingstation.co.uk/products/Belts