> how did you attach the flash memory?
Read "Designing Embedded Hardware"! It's in the book! :-)
> how did you get it so small?
Blood, sweat and tears. The really difficult bit is actually debugging hardware that small.
>Why is a PIC smaller than an HC11?
The PIC16LF73 is a physically smaller processor, and requires less support than a HC11. Further, the PIC (and the AVR processor) uses considerably less power than a HC11. The datalogger uses only an average of 12 nA! The downside is that there are other processors that are much easier to write code for than the PIC. It has only a limited architecture and often considerable time is spent working out the best way to write a piece of code, rather than on developing the actual agorithms used. I'm looking at moving the design away from the PIC. If/when the dsPIC is ever released, that is a likely candidate.