Chain size, 420, 520, 530 etc...... depends on the bike size and sprokets you use.
Chain length, again depends on the bike, gearing, length between sprokets etc.....
Not sure about formula's or a "rule of thumb" you can use. Most sproket manufacturers will offer gearing charts and chain length info on their web sites.
it is an 04 crf 450 and i plan on using a 13 front and a 50 rear. stock is 13 48 do i just add 2 links to that to get the correct chain. any recomendations on the gearing or chain
Most places will sell you around 120 links, standard..
This will be a few links longer than you need. Setup your bike how you like the rear wheel, and measure your chain, then cut the excess links out. Be careful so as to not remove too many links.
It's not rocket science.. Simple counting will do.