are 19s too big?

The width of the wheel or size of the tire have nothing to do with spacers, its the offsets. If you buy wheels made for the boxster and have the correct offsets you wont need spacers. My company had them universal for 996/986 so I did need a 7mm spacer on the rear. If you buy a proper set that are custom made for a boxster's offset (BBS, O.Z., Fikse, etc...) you wont have any worries. I hate to go expensive on you, but tirerack has some good stuff. If not, ebay rims seem to be just fine surprisingly. I use a 275 rear BTW and a 235 front.