Dont know largest size as it depends on manufacturer as to actual tire size, but I have 559×40 (26×1.50) Schwalbe marathon plus. The tire measures at 1.5 inches wide with 0.5 inches clearance between tire and fork on both sides. The tire measures 1.5 inches from rim to top of tire with 5/8ths of an inch clearance between top of tire and fork - see pic.And the rear has more room than the front fork.