What size is the exhaust on a 1990 ford f-150?

There's no single answer to the exhaust size on a 1990 Ford F-150. The exhaust pipe diameter varied depending on the engine size and options chosen by the original buyer. A smaller engine would likely have a smaller diameter exhaust than a larger engine. You'll need to determine the engine size of your specific truck to get a more accurate answer, or physically measure the existing exhaust pipe.