Test it on himself

Draco snorted in disbelief. this really must be a joke. but then he thought to himself if it was real then he could benefit greatly.

he looked at his crotch. before he was proud of his cock and its 8 inches of proud pureblood man-meat but if this book was real then he could change it up. he picked up his quill and began to write on a blank page

Draco Malfoy's cock is 12 inches long and 3 inches thick and has the stamina to go with it.

immediately after ending his sentence, Draco felt a strange sensation in his crotch. he watched as his bulge grew and grew until it tore through his pants and his newly enlarged cock stood proudly and his balls had grown to accommodate.

"Holy Shit! It works." he exclaimed.