exact header tube size and length is hard to determine. similar formula as an intake manifold except the other end.
A 4 into 1 header is like a single plane intake,it will make more power at a higher rpm than tri Y headers.Try Ys act like a dual plane with the bottom end increases starting at a lower rpm and rolling off peak powers sooner.
Tube size,smaller tubes scavenge the head sooner but will not have enough volume at higher RPMs or on bigger displacement engines.
some engine builders will generalize and say you need 1 5/8 primaries for 375 hp or less and 1 3/4 primaries up to 500 hp,after 500 hp you might need 1 7/8 primary pipes.
this type of statement is ok at best,or a starting point. There are a lot of ponies left on the table because people buy a $200.00 set of headers that fit 5 different cars,then spend 10k on stroker engines with big cam/heads and wonder why there engine makes ok but not spectacular power
choosing perfect headers is very complicated and almost always over looked