Thank you both for your replies, all good to know, you learn something new every day!
I forgot to mention in my last post but my snake dimensions (I took her out for a gentle check) are:
3.5cm thick at the widest point (quite a bit bigger than the small rats I think)
297g weight
60cm-ish long (it's hard to straighten a snake )

Given this, should I maybe move up to the next size of rat?

Also, I distinctly remember the reptile shop owner saying that they were mice but my reciept does indeed say "Frozen rat fuzzies/fluffs", what's the next size up from that?