Are you going to put any borders around the panel or just keep it like it is?
In general, your backing needs to be at least 3" larger on all sides than your quilt top. (I usually go 4"-5" larger.) If you don't add borders to change the size of the quilt top, your backing must be at least 30" (24"+3"+3") wide by 51" (45"+3"+3") long - and I'd make it a little bigger than that, if I were you. (The extra fabric that you'll trim off at the end works great in scrappy quilts.)