What size screws can this tool extract?

Should be listed on the web site.

I believe the packaging states that it can remove up to 4" screws.

The screws that I used this product on were #8-#10 and 3 1/2" inches long. In the cases where the screw just could not be gripped or in the case where it had sheared off inside over the years due to torsional forces on the deck (I have since learned that screws are not designed for deck uses and would not meet today's code.., those I simply had to try and grab at the sheared level.