There is a way to do this with more certainty though, as opposed to just guessing. Don't guess, especially not with a garment!
I looked through the pattern and found the stitch counts of each piece at the bust area. I'm not going to list the actual numbers here to protect the integrity of the pattern, but for example, let's say each front was 30 sts for the smallest size and 60 sts for the back piece. Subtract 2 sts from each piece to account for seaming the sides together and picking up sts along the front for the button bands--28 and 58, respectively. 28 + 58 + 28 =114 sts. If the pattern gauge calls for say, 3 sts per inch, that would turn out to be 38" around. Add in a 1" buttonband and you've got a 39" finished bust measurement. But if your gauge is 3.5 sts per inch, you'd wind up with a 33.5" bust (32.5" body + 1" buttonband). Big difference! That's why gauge is so important.
While I would have aimed for the 34.25" size if I was on point with my gauge, with my new gauge I'm going to be following the directions for the 31.5" size. If my math is correct, I'll wind up with a 34.25" anyways, which will give me just enough ease with my 33" bust. Yay!
You can use this technique even if your gauge is on point, but maybe your personal measurements don't jive with the sizes offered. If it's a sweater worked in pieces and seamed, you can even combine pieces from different sizes to get a custom size, like following the directions for size M for the cardigan fronts and size L for the cardigan back. Some adjustments might need to be made in length-based areas, like the body length of the piece or the armhole depth, since those measurements don't always stay consistent over multiple sizes. You'll just have to make some design choices and maybe rework some increase or decrease sections to fit your new franken-garment. If you're used to modifying patterns, you have the skills to tackle this technique!