Sometimes they will give up, sometimes they will get it down.
If they get it down, there's a chance they regurge if they eat something too big, and you don't want that. Better to feed multiple small prey than something too large.
If you don't feel comfortable eyeballing food sizes yet, see if it's possible to return a rat to to store if needed.
If it is and you really want to know, buy a rat, bring it home, weight the rat first. In your snake's case, if it's around 60g or less (probably bigger, but don't go too big) he can get it down easily even if it doesn't look like it. If not, return the rat.