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according to this calculator you could run a 245 / 55 / 15 and your speedo would only be 0.2% to fast, as far as the largest 15 that will fit under the rear, someone else will have to step in.
Do you mean tallest or widest tire? While the 15" wheel should bolt on, the problem will be wheel offset and the amount of space between the rear suspension components and the inside edge of the tire.