It looks to me like the sweater is meant to be somewhat loose fitting, but it does have waist shaping so even if you made the largest size, the waist shaping would likely still make it flattering. You'd kind of have to choose the largest size in order for it not to be too snug at the bust.
Just looking at the stitch count at the waist shaping -- if you made the XL, after the waist decreases you would have 51sts. With the gauge of 12sts/4" that would give you a waist measurement of about 17" across the back, or about 34" total. You'd have to decide whether that would work for you or not.
Just another though of something you could do.....you could start off the pattern using smaller size (if you did the size small, it would be about 30.5" around at the bottom edge and about 26.5" around at the smallest part of the waist) but when you get to the bust area, keep increasing until you get to the stitch count of the XL.
Sorry, I'm feeling long winded here :teehee: , but I also just noticed that it says under 'Gauge' that it is worked in a loose gauge (worsted yarn on large needles) so it is very stretchy -- in other words, you'd probably be able to get away with the size large, even for the bust area.
Good luck! :muah: