Right i recently bought a 6pot AP caliper of ebay as it was dirt cheap and brand new

http://cgi.ebay.co.uk/ws/eBayISAPI.d...m=250200809742

whats the max size disc i can use under a 18" comp mo rim?

and wheres the best place to get the matching caliper?

the part number is CP3796-26s4

also what size rear brake should i go for?