It might be interesting to produce a surface-mount variant of the board. People could then practice their SMT soldering, or get a fab-house to load the boards on a Juki. 0805 would be a reasonable comprise on hand-solderability and board space. You could get a SMT LDO, AtMega, MCP23S17, MCP7940N, MAX3232 along with all the decoupling caps and resistors. That would just leave the headers and connectors to be hand soldered.
It might be interesting to produce a surface-mount variant of the board. People could then practice their SMT soldering, or get a fab-house to load the boards on a Juki. 0805 would be a reasonable comprise on hand-solderability and board space. You could get a SMT LDO, AtMega, MCP23S17, MCP7940N, MAX3232 along with all the decoupling caps and resistors. That would just leave the headers and connectors to be hand soldered.