2 inner tubes? Take one, thats all i take. Was none today(wished i took one though, got a puncture
)
You wont need a big one, i wouldnt bother putting your mobile phone in the saddle bag, i did and forgot to zip it up, one lost phone
Mine is pretty small, but with the multi tool on the bottom, inner tube, then tyre levers, its all in a small package.
Capacity wise i dunno, but have look in
Evans on the 16th August, they have a big sale on so you should be able to pick up a bargin.
I have the medium sized Ortleab(spelling right?) saddle bag, the hard case one. Does me and has abit of spare room.