our standard connector for larger power is the ceeform(IEC 60309) a 125A 3 pin would be about 4" in diameter and 12" long
But note that in practise the 125A Cee is actually good for about 80A when run with the usual 25mm square cable, they can and will overheat if you try for 125A on all phases for more then a very few minutes, and this is before you consider the possibility of triplen on the neutral (The lighting guys are finally getting better about that). Usually better to top out at 63A on Cee and then switch to powerlock connectors or lugs.