What size pocket hole screw for 1.5x1.5 miter?
The size of pocket hole  screws are determined by the thickness of the wood or material you are joining not by the type of joinery. I.E. butt joint or miter joint.
There is a guide that comes with the kreg system that will tell what size screw you need for the thickness of wood you are working with. (You do not say if your wood is 1.5 thick or 1.5 wide, it is the thickness that matters)
Note: There are other pocket-hole screw systems, this happens to be the one i purchased.
For 1 1/2&quot; material it is a 2 1/2&quot; screw