on the standard foci the pad carriers are cast into the hub so you physically cant go any bigger!!! if you dont want huge expense then get uprated pads and disks and leave it at that.

or

if you really want bigger like me...get st170 hubs and callipers. most of the time i see them go for about £250 although i was lucky and got a set the other day with worn disks and pads for £100....frigin steel!!

though obviously there s more work in fitting them and the disks and pads cost more.

oh and the min wheel size you will need is 16"