The ebay stuff has a huge exhaust housing as its a load of ripoff shite from a diesel truck.
Do NOT get ebay junk.
You dont need a huge hot side either, best results i seen was a good 650odd bhp and lots of torque using a GT35R with a .82 exhaust housing, ie the middle sized one.
RB26 manifold wont fit a RB25 head, and no reason to do that, RB26 twin turbo setup just means fuckloads of pipes and grief for nothing.
Ring up InvoAuto and speak to Tommy "FIA aprooooved, innit" Chung, im sure he does stainless single turbo RB25 manifolds for about 250quid that will enable you to fit a big single turbo and external gate of your choice.
If you wanna stick to standard manifold then its a T25 flange and biggest you can fit that i can think of is GT3071R-WG and thats good for 450bhp.