How to find exact size of spare part?

[jsmoke]

I was looking for an axle on buycarparts. So you put in your reg and bring up axles and I find all different lengths, I get, 910mm, 939mm, 915mm, 912mm, 928mm. Is it possible these lengths were just measured by hand and they are all the same size?

If not how would I find the exact size for my car?

 

[Begbie]

measure it?

It'll be per engine, so find out what engine relates to what size etc.