If the tokens are downloadable, you should be able to download the set from the Marketplace page where you purchased them. The easiest way to find out what graphics editor is the default of your system is to double click on the downloaded file (almost certainly a png file). In most cases, if you are on a Windows PC, the editor is the app called Paint. On a Mac, it will likely be Preview. When it opens, look for a tool or command called "crop". It should allow you to select an area of your token that you want to fill the square. Try to keep your crop selection square instead of rectangular. In most programs, holding Shift while dragging will accomplish this. Save the cropped png to your hard drive, and then drag the file to your roll20 game table. Size it to one grid square and the image should fill more of the square.