Well, it depends on what you want to bring or put inside the bag.
A Lowepro Nova 2AW will allow you to put in a lens (70-300) or flash (Sigma 500DG Super) with the 300D attached with lens. Any bag that is lower than the Nova 2AW in capacity is not advisable. The Lowepro Nova 3AW will allow you to put in the 300D with lens and another 4 more items ie lens/flash (70 -300 mm/Sigma 500DG Super) I have both bags and the relevant items that I have inserted.
You should have a rough idea from the above. Cheers.
Oh yea... glad this thread popped up. I'm also looking for the same thing, except that the bag has got to accomodate a 300D with grip. A couple of lenses (28-135(with hood),50,18-55) and a 500 DG Super.
Can the Microtrekker accomodate this?
Would most likely prefer backpack rather than sling.