I saw this antique wearable abacus recently and thought "How small an abacus can I make?" Answer is "Not quite that small, but pretty tiny."
Chinese abacus ring, about 12mm x 7mm (0.5" x 0.36"), that inspired me.
I cut some thin slivers of wood, drilled tiny (#66 drill bit, .033", .838mm) holes, and then glued into a frame shape. To hold it while the glue dried, I stuck it to blue painters tape. My smallest seed beads are 1.5mm, so I drilled holes 2mm apart. Final size will be about twice as wide as the ring.
I stained the wood and glued a brass angle iron on for reinforcement. I'm using colored jewelry wire.
I'm using a bead tweezers I cut from a coffee stirrer (see first picture, next to blue tape). It flexes to grasp beads pushed in, then the wire catches them to remove. Grid on ruler is in tenths of an inch.
Smaller than a 4x2 Lego plate. Functional abacus with ten rods of seven beads each, brass reinforced wood frame.