[Ulzgoroth]

So if you look at Low Tech, it's got somewhat clear answers to this embedded in the shield descriptions...

Large shield: "They cover the warrior from chin to shins,"
Medium shield: "covering from chin to thighs"
Small shield: "rarely longer than the forearm"

I can't particularly reconcile that with your numbers, but it might be worth asking whether you're sure you've got the right scutum. From looking it up, it sounds like that's a rectangular medium scutum (DB2), not the earlier eliptical heavy one. I don't have the hoplology scholarship to say whether Low Tech's distinctions and numbers are sound, but they are there.