Personally, if I thought a snake needed a "bit more", I'd prefer to feed the same (smaller size) but slightly sooner (meals closer together), rather than upsizing to a medium rat, because they can digest one easier than 2. (2 rats have a larger "surface area", ie. a larger proportion of undigestible fur in that one meal) So if my snake was eating every 2 weeks, for example, I might feed it every 10-12 days; you always want to allow plenty of time for good digestion too. But depending on your snake's size & other factors (age & activity levels, etc) a medium isn't necessarily wrong either. Snakes are pretty flexible.