What is the likelihood of colon cancer if you have a 1.5cm polyp?

Depends: Here size doesn't matter. It is the cell type seen by the pathologist under the microscope that is important. Hopefully yours is benign and it sounds like you were fortunate to find a smaller polyp that could be removed successfully.

Biopsy result.: If this is all you have in your colon on your colonoscopy, you then need to have it biopsied to determine if it is cancerous or prrecancerous. This can also tell the extent of spread of cancer if it is present. Usually a solitary polyp like this is unlikely to be cancerous, but you don't know unless a biopsy is done so see your doctor and get that biopsy result if not done already.