CustomFit uses your gauge and measurements to determine how much yarn you'll need for your sweater. If you haven't swatched yet, how do you know how much yarn to buy?
Our first recommendation is the app Stashbot by Hannah Fettig. Choose a gauge, a chest size, and a type of garment, and you'll get a rough yardage estimate! Ann Budd’s Handy Book of Sweater Patterns or her app KnitHandy also accomplish the same results.
If none of those work for you, you can do some research on Ravelry. Look for sweater patterns in the style you're making, and check out the yardage requirements for approximately your size. You can also look at projects and see if there are any FOs in your bust size and see how much yarn they used. If you've got a LYS around, they may be able to give some guidance as well.
Some folks also purchase one skein to swatch, and then purchase the rest once they have a yardage estimate, which may work for you.