Well, for iTunes go into Preferences - Advanced and change the location of the iTunes Library - I created a Music folder on my external for this. One thing you need to know when doing this - if you open iTunes while your external drive is not connected, and add anything - or it starts auto downloading any subscribed podcasts, iTunes will change this location back to your internal drive and you'll end up having to move these and go back and change this location again.
Space size for the bootable backup partition is going to be an individual thing.
for myself - looked at the amount of space I was using on my internal not including my music and movies - was about 35 GB at the time - made a judgement call and decided that I would probably never have more than 65GB used on my OS X partition, so, created a 75GB partition for my backups. Need to take into consideration any large apps you intend on installing in the future.
One note to realize about my particular setup. The way I use it, I have no backup of my iTunes library, nor any of the movies I have moved to the external drive. So, until I get a second external, if the one I have goes down, all of that stuff goes bye-bye. My music can be recovered from my iPod, but the movies will be history.
My movies are recordings I have made from TV or back ups of my DVD's (of which, I am probably in the 600 range right now). My intention is to eventually have all my movies available for streaming from a media center rather than going and getting the DVD from the place I store them.