This is my offer for the smallest design on the MCP23017 I/O expander in bi-directional mode:
It's possible to smoosh an SOIC with 0603 SMD down to 23mm x 107mm (0.9in x 4.2in), with no support holes (I slide them vertically around the enclosure perimeter. The headers should make for easy access.
Circuit looks like this:
Hopefully I didn't do something real dumb, like on my prototype (A0-2 wired wrong, can only get 111 without hot-jumping wires)
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