Hi There

i am making a new petrol tank and wounderd what size pipes i should have for the filla neck, breather, etc

starting with the filler neck, do you intend to have a sort of aero type fuel cap? if so, the inlet to the tank will really need to match the funnel for the fuel cap assembly. 2", or 51mm seem the most common.
http://www.merlinmotorsport.co.uk/p6927/FLUSH-FITTING-FUEL-CAP-3.75-O.D.-WITH-LOCK/product_info.html
http://www.merlinmotorsport.co.uk/p1778/FLUSH-FITTING-FUEL-CAP-FUNNEL-FOR-2-HOSE/product_info.html

or if you want a screw on cap straight on the tank
http://www.alloyracingfabrications.com/cap-and-neck.html

as for outlets, easiest to match the same as the inlet to the carbs/throttle body fuel rail so everthings the same.
8mm or 10mm is common

breather, 8mm is fine

basically, really depends on everything you're trying to attach to it!

mine has 8mm outlets, and the carbs have 10mm inlets. so everything keeps changing size, and it gets quite confusing

50mm for the filler.
8mm for the return
8mm for the breather
8mm for the outlet if carbs, or if injection check the size of the pump inlet. External FI pumps often have either 10 or 12mm inlet so match the feed from the tank to that.