The front-to-back dimension is the key. Length (top-to-bottom) is a non-issue, width is almost a non-issue. The problem is that that M4/16 magazines are too "long" (front-to-back).
The RS Type 97 Series magwell is definitely on the shorter side (front-to-back). You're looking at 68mm, I'd go 65mm to be on the safe side. I know 65mm works from first hand experience.
P.S. The magazine catch on the RS is tough. Unless you depress it hard when you insert or remove your mags, it'll eat the side of your magazines down in no time flat.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."