I am not sure the size windows still needs to be, it also will depend upon which windows. You could start with 30GB to 50GB and if not enough room left for expansion expand the partition with gparted. My old WinXP with lots of stuff over 5 years is 30GB, but I have moved most data to a shared partition. You then can partition the rest of the drive as you please. NTFS for anything you may share and ext3 or ext4 for Ubuntu stuff. You could leave some unused until you know and then either expand or add another partition.
When you plug your Ubuntu drive back in as master it should boot normally. You will need a map type entry to make windows think it is on the first drive. If not in second drive first partition sdb1 or grub (hd1,0) you will have to adjust entry:
Add before or after the automagic area in /boot/grub/menu.lst
to backup and then to edit:
Code:
sudo cp /boot/grub/menu.lst /boot/grub/menu.lst.backup
gksudo gedit /boot/grub/menu.lst
title Microsoft Windows on sdb1
rootnoverify (hd1,0)
savedefault
map (hd0) (hd1)
map (hd1) (hd0)
chainloader +1
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