What are the biggest wheels and tires..

IMHO and experience with my 15x10 inch rims on my '72 Duster is the 10" width really limits the availability of tires that will fit on them and also have enough room in the wheel wells.

Before you decide on 10 inch width go check out the tire sizes you have in mind at: http://www.tirerack.com/tires/SearchTires.jsp

You'll see that most tires in the 255 to 265 sizes are not recommended for rims anything larger than an 8 inch rim. At best only one or two tire makers now make tires in those sizes that fit 10" wide rims. BFG makes a 265 that'll work on 10" wide rims but they SUCK if you need a tire for traction and are speed rated at 118 mph.

Another thing you'll find out, most tires in the 255 & 265 sizes for 15" rims will have low speed ratings.