A sweater is a garment, so it's important to understand ease and your own measurements so that you can make accurate decisions about which size to make. When a pattern lists sizes as 1, 2, 3, etc... you can generally think of them as S, M, L, etc... However, it's more important to measure yourself so you know your actual measurements, then refer to the finished measurements in the pattern so that you can compare those two sets of numbers and make sure that the size you want to make will: a) fit you, and b) have the amount of ease you like for a comfortable fit.

Refer to these two blog posts to assist further —

How to Choose the Size for Your Cardigan

Knit Better Sweaters (Ease)