I've never made a start buttons...but it seems to me that you should download one of pelaird's start buttons, open the zip file and import it into Photoshop (or similar). Check the image file type (.png) and enable rulers so you can get the dimensions. Check the image size, as well.
OR, you can look up pelaird, and drop him a pm (I'll do that for you)...which might be the easiest choice.
Good luck!