Let's take this in order...
1) I personally would make three partitions: one of perhaps 50GB for the WinXP and associated programs/data, one of perhaps 1GB for the OS/2 system partition, and the rest for OS/2 programs and data. Partition C: for WinXP, D: for OS/2 and E: for OS/2 data and programs.
2) I personally would not
use the OS/2 boot manager, as it's not necessary and you can achieve the same thing with the WinXP boot.ini. Also, the OS/2 boot manager requires an extra partition. I found this inconvenient and wasteful in the past.
3) The danis506.add is not part of the OS/2 installation, and you'd have to get it from http://hobbes.nmsu.edu/
, which is a good source anyway. But OS/2 (system and programs) are nowhere near as large as the Windows variety, so you can use a much smaller partition.
4) I would install WinXP on C: first, then the OS/2 system on D: If you reverse the order, the WinXP installation will overwrite the MBR and you'll have to use sysinstx to fix it (and of course, WinXP won't boot from D:). All bets are off if you use the recovery CDs - no idea what they do, and I personally wouldn't risk it. If I remember correctly, you'll probably have to use the sysinstx program to make your partition D: bootable anyway - but I can't guarantee for my memory on this.
Putting the OS/2 system on a separate partition has the advantage that you can change it (for example to eCS) without having to re-install or modify any of your programs.
By the way, definitely use HPFS as the file system for OS/2 - that will make the OS/2 partitions inaccessible for the WinXP system, but the HPFS system is much
more stable and resistant to damage than FAT.