Detect if the user used the wrong --size format
Detect if the user used the wrong --size format

fixes #9

portgasd666/pageres,elgubenis/pageres,sindresorhus/pageres,RyanTech/pageres,JuanitoFatas/pageres,Ahmed-Badawy/pageres,elgubenis/pageres,portgasd666/pageres,Ahmed-Badawy/pageres,sindresorhus/pageres,sindresorhus/pageres,JuanitoFatas/pageres,RyanTech/pageres