You're in luck. We've got some old, long-gone GP15s on the roster that haven't been 'written-over' with renumbered units yet so they're still on the engineering tables. Truck Centers on BNSF's 1977-built GP15-1s (the same as a "T", just minus the Turbo) is 357 inches -- 29.75ft.
If you're curious, Height is 186" and Width is 124"