Aftermarket up-pipe and turbo spool
 

what effect will a bigger diameter up pipe have on turbo spool? will it spool quicker or slower?

Slower - the collective velociy of the air will be reduced as it spreads out to fill the cavity so it will be moving with less inertia when it hits the turbo.

Does it not depend on how much air you also want to shift and whether a certain size actually causes choke? Not sure if this is applicable to up-pipes though.

Yes and opening up the bore will decrease the force with which the air strikes the turbo and that will slow the spool. As you open up the bore up you're allowing gas molecules to move more in other directions so fewer of them will be striking the impeller simultaneously. Your turbo will choke long before the up pipe will. Also, opening up the pipe is an excellent way to supercool the exhaust gas before it hits the turbo by creating a nice big pressure drop...

That answers my question then! :D