How small does it have to be? Chibi Scheme does syntax-rules in 174 lines of Scheme, layered over explicit renaming. Indeed, Chibi does essentially all of R5RS in 4822 lines of C + 708 lines of Scheme.
— John Cowan
Scheme 9 from Empty Space is also very small from what Nils said.
(via this thread in r6rs-discuss, I have the email but can not find the archive of it)