Welcome to the forum and congrats on your new snake!!
It's great that you're taking the snake to the vet for a 'once over' including sexing it and an accurate weight.

Some forum members feed 10% of the snake's body weight (in grams). Some forum members feed every 5 days until the snake is at least 200g. Some forum members choose to feed rats and others choose to feed mice. Some choose live feeding, others pre-killed and still others F/T. There are a lot of ways to get a nutritional meal into your snake and it will really depend on what is good for you and if your snake has an opinion

I think, were I in your shoes, I would get the weight from the vet on Tuesday and then feed on Wednesday; allowing your snake time to de-stress from the stress of the vet a appt. You'll know what size to feed and you'll have had a chance to consider what prey and what type of feeding method will work best for you (live or dead).

Some things to think about:
It can be difficult to change a snake from eating mice to eating rats. If you choose mice, keep this in mind!
It can be difficult to switch a snake over to Dead after feeding Live.

Lastly, we love pictures around here and would love to see a picture of your snake!! Feel free to ask any questions or concerns you might have about its environment as well. As many different ways as there are to feed a snake is about the same with environments- save for a few basic necessities!!

Again,