If you are working off the latest trunk, MEMISOSIZE is automatic. In earlier versions, the actual size is reported at the end of the build, and you need to set MEMISOSIZE to be >= to that.
MEMISOSTART needs to be after the end what the canon firmware loads starting 0x1900 and later. You find this by examining the disassembly. This normally includes some data copied from ROM, followed by a large chunk of memory initialized to zero (i.e. initialized data and BSS for the firmware).
For dryos, the number you are looking for should be in the first function at the start of ROM, after the "gaonisoy" string and the stuff that fiddles with CP15
RESTARTSTART needs to be after where your CHDK is initially loaded ( i.e. >= 0x1900+ your memisosize.)
Assuming the LED addresses have been found for the 590, you can light up an led early in the bootprocess to make sure your code is getting loaded at all.
edit:
Looks to me like copied ROM data ends at 0xDC48 and zeroed data ends at 0xA5304
Note that the
a720 sub/*/boot.c uses the equivalent of these values (canon_data_len, canon_bss_len), and has a whole bunch of addresses and ASM which you can't just copy.
