Your snake sounds young and a small rat should be to big for it. If you want to switch to rats you sould get a rat fuzzy of maybe a rat pup, at the largest.
If it was me I would wait three days and then try to feed again.
Here is a link that has a good chart showing different feeder sizes.
http://www.google.com/imgres?imgurl=...Q9QEwBA&dur=60
Most suggest to use a feeder that will only leave a slight buldge in the snakes thickest section.