I'd go with the bigger size, but since you've bought the fuzzies, feed her two. Mine started on pinkie rats in April and is getting one small rat every 7 days now he's 8 months old. I don't weigh them but go on size. I think general rule of thumb is something the same girth as your snakes roundest part of their belly.
Edit: pic to keep on topic. I fed him last night: such a little fatty
If you are feeding something that is as big as the largest part of the snakes body, it's far too small.
I always started feeding hatchlings on 'fuzzy' rats. About 14-20g. From this I would progress pretty quickly and at 9-12 months age would be on small adult rats once a fortnight. I always prefered to feed larger meals less often for womas and all of mine had no issues with condition or size. If they were starting to get fat I'd back it off. If they were maintaining a good weight I'd keep feeding as I was.
The main thing for womas is that they maintain a good body shape, like an arched window. Curved at the top, flat on the sides and pretty much straight across the bottom. If they are looking plump around the tail, they are too fat. Feed smaller meals once every 3 weeks till the weight drop off. They are naturally quite lean in the wild.
