Originally Posted by
nubcake
As for gearing the lowest you would be able to fit is a 28 tooth cassette and there is no smaller chainrings out there that I am aware of.
I don't know about the Tiagra RD that the OP's bike uses, but I've seen people using 30-tooth IRD cassettes successfully with higher-end Shimano derailleurs. It wouldn't surprise me if you could run a 32-tooth cassette, though you might have to swap in a longer B-tension screw and shift quality would probably suffer.
The smallest Shimano-compatible chainring I can find for a 110mm BCD crank is
33-teeth.