I’ve seen people (including @tea) make extremely effective use of bucklers 12" or even smaller.
I’m remembering a short sparring match years ago (maybe 2007) when I had a 42" sword and 20" round shield, and my opponent with a tiny buckler (eight or nine inches?) managed to hit me thrice on the left ear within a few minutes with his crowbar physrep of maybe 20-24".
That’s the person I paid to make me a buckler when Empire started: I wanted a trapezoidal one with a wave-shaped cross-section: see Tips on League combat - #15 by Canashir
It’s less than the maximum allowable size. The size is a compromise: I wanted it small enough to that I can carry sword and buckler on my hip as a matter of course without it being too cumbersome or inconvenient. So its dimensions are
9+3/8 inches wide at the top (238mm)
7+3/4 inches wide at the bottom (196mm)
11+11/16 inches tall (297mm)
Roughly 14+3/16 inches across one diagonal (362mm)
Roughly 14+1/8 inches across the other diagonal (360mm)
It’s painted with latex and then a protective coat of clear isoflex primer.
The grip is wrapped in leather.
Apart from that, it’s made entirely from layers of canvas and foam held together with glue and possibly a bit of sewing.
The foam is closed-cell polyethene, probably Plastazote.
The glue is Evo-stik epoxy (the solvent-based kind, I believe).
I vaguely remember the canvas looking like denim in a work-in-progress picture I saw.