Help with sizing up

[IamSavage]

Hi all and merry Christmas need some help with some bits. What it is, is my farther in law is wanting to make from scratch a rc fire truck witch will be fully working even pumping water and so on, so what I am asking is what size crewler diff and axles r as he wants to have axles like a big rig if u get me

So if I got say a 1/10 crewler front axle need total size in say mm or inches


Any info would be grate

Thanks all