It can also depend whether the rain suit is designed as an "oversuit" or as a "rain suit". Over suits are often fit larger such that a "large is made to fit over a large". Rain suits are typically designed to run true to size, so a "large fits like a large".

I tend to take my gear and go try things on before I buy layering kit.

One nice thing I tend to keep in mind - if it's raining, it's probably not very cold, so I don't have to have a big sumo sized rain fly. If it's raining, it's probably warm enough that only a thinner outer layer is needed, so even if the sizing between the two is a little tight, it still goes on a lot easier than packing deep cold outer layers into a rain suit.

Also, keep this in mind: YOUR BOOTS MUST GO THROUGH THE PANTS! Packable rain gear will typically come out while you're already dressed and in the field, so you'll already be wearing your boots. Unless you want to take them off in the field, make sure they'll fit through the pants. I like taking my boots off in the field less than I like mud inside my pants, so I run my boots through instead of taking them on to put on the pants.