We know so little of the post Roman era. Procopius mentions small shields strapped to the upper left arm. Plus, the Comitatus re-enactment group (
https://www.comitatus.net) has attempted to use a number of late Roman type weapons and armour. As John Conyard of Comitatus says "I have experimented with a small shield, 450mm in diameter, strapped to the left fore arm. The small shield allows the kontos to be held in both hands, and allows for the use of bow even when using a western release. The shield boss is retained to allow the shield to be carried in a more conventional manner."
So there is no reason to believe that foot archers could not have operated with an even smaller shield/buckler/targe (with or without boss) strapped to the forearm as they used it. Assuming average height of 5 ft 6in is represented by a 30mm miniature (so 28mm to the eyes etc), then a 450mm shield would be 8mm wide.
Whether Romano-British archers used such shields is just not answerable. If you like that look then go for it. My own RB archers tend to have a mix of no shield, 6-8mm bossless bucklers, and 10-12mm shields (slung over the back) to give a feeling of less uniformity.