What is the biggest motor to put in a f150?

There's no single "biggest" engine you can put in an F-150 in the sense of displacement, because it depends on what you mean by "put in." You could theoretically fit an enormous engine with enough custom fabrication, but it wouldn't be practical or street legal.

The largest *factory-installed* engine Ford ever offered in an F-150 was the 7.3L gasoline V8 (found in some older models). However, even that isn't the *physically* largest engine you *could* squeeze in. Aftermarket swaps are common, but they require significant modifications, custom mounts, and potentially major chassis alterations. Someone could theoretically install a much larger engine from a different vehicle, even a diesel, but the cost, complexity, and legality would be immense. The resulting vehicle would likely handle very poorly and be far from roadworthy without substantial engineering and fabrication.