That sounded really cute!
There are two ways you can decrease the size of the blanket that I can think of right now. One is to use a finer yarn smaller gauge needles. Second is to reduce the number of cast on stitches by half. It depends on how your blanket is designed. If it has a pattern then make sure you have the right number to fit the pattern as well as the borders (if there is any).
Thank you for visiting and question. Have a nice weekend!