You don't measure it. Take it to a machine shop and get them to look at the crank bearing saddles in the block and the removable main bearing girdle and the rod big ends to see if they are suitable for a new crank, then just use the original size bearings if everything is ok. I would have the rods reconditioned and do the bores and pistons and rings and valves while I had it apart...

Not responsible for advice not taken...