You can get a kitchen scale to weigh your snake. They are cheap on amazon.

As for the feeding its usually recommended to feed prey that weights 10% of your snakes weight. Since we dont know the weight we switch to eyeballing it. You want to serve a prey item that is around the same size as your snakes girth at its widest part on its body.

When feeding more then one feeder just serve the first and once the first has been completely swallowed and worked down through the neck the second can be served. This should not be need though it you are offering the proper size prey be it mice or rats.