You don't sound stupid because you have asked a valid question. I am a little confused by your account of the previous owner if the snake is 18mths old? "He had fed her 3 times, about once every 3 weeks." Are you the third owner this snake has had?
But basically I would have graduated up one size mouse so from a fuzzy to a weener and gone with that for a few feeds and then move up to half grown/subadult. The belly bump is a good guide to start with, then look at how much bigger the food source is rather than just it's weight. Mine was only on weeners for about 5 feeds and then was ready for the subadults for a few months.
If it spat it out then it is probably too big. Keep your leftovers in the freezer for the time being and get hold of something a bit smaller for now, even if you feed it two in a sitting. I'm fairly new to all this too but it has worked well with Skittles, I haven't needed to offer two items to her at all, and my mate with years of experience was very impressed when he saw her yesterday.