A LEGO stud is approximately 5/16" wide, or 8mm. I believe the longest solid axles that come in the kit are 12 studs long, or 3.75". LEGO makes a lot of other wierd pieces, though, that aren't in usual kits. I have a 32 stud long axle (10"), which is the longest I know of. You can buy them online at
bricklink.com, but I don't know the rules on LEGO pieces for FTC.