I went big and bought the FI.13 bibs. Rode them for the first time this week. I'd never worn bibs before, so that was a new experience as well. Trouble for me is that I need what would be an XL in Assos because of my height (5-10") and a large or less because of my waist (30-31"). I got the large and they are definitely tight across the suspenders. Other than that, they seem perhaps a little more comfy than my mid-level Pearl shorts.
Bibs feel funny, though. Without the waistband, it feels like you're wearing a barrel with suspenders. Suck in your gut and it feels like you can air out your, um, parts.