what are the biggest ford calipers that will fit on an escort GTi? and where might i get some

Standard wheels by the way 15"

 

as far as i'm aware it's the stuff of the escort cossy set up

 

Yep or there is a mondeo setup with the same size calipers just the mounting holes are slightly smaller and need to be drilled out but you will probably find the mondeo ones alot cheaper than the cossies for obvious reasons. Also you will need a spacer to centralise the caliper on the disc probably about 4mm.