parma

[caiusbeerquitius]

Are we concluding that the relative small size of the shields in the gravestones is an accurate depiction of the size of the actual shields?

Rather not:

If it in fact was smaller may be discussed, since it quite often happens, that shields are displayed way smaller than the findings suggest they actually were.

same size as infantry shields.

Rectangular shield belonging to the aquilifer(?) standing on the floor in front of them

same size as the infantry shields

slightly smaller than infantry shields

about the same size as infantry shields

oval shields of the same size as the infantry shields

If I combine this with the other evidence above, I´d say that a curved (like the Kasr-El-Harit) or dished (like the Dura Europos) shield board about the same height as a rectangular scutum, or slightly smaller, would be rather what we are looking at here.

And:

Some of those look so small they'd hardly be any protection at all, at least to me. Could they not be shown small simply because of artistic convention?

Difficult to say, but basically I would agree. Look at The helmet in the upper right corner of the first tombstone in my post above. It will hardly fit the guy´s head. Displaying shields way smaller than they were has a good tradition in Greek, Hellenistic and Roman Art.
BUT if shields actually WERE smaller, it´d be hard to tell the difference...