This is just me, but the best resource I've found for getting a feel for what a bag can hold is to look it up on
http://cambags.com. This is not a commercial website selling you anything, but is a crowd-source review site for camera bags, and folks will post pics of the bag fully loaded. Since nobody loads their bag the same way as anybody else, you get more of a feel as to whether the bag might fit your needs. It may not contain reviews on the latest bags, but it's got a pretty fair sampling.
The main thing to understand is that the "perfect" camera bag is an endless quest, that often ends in moving from one bag to another as your needs change and your pile of gear grows, or to have multiple bags for different situations. Keep in mind that the gear you have now may not be all the gear you want to lug around over the next year or two. Give yourself a little room to grow if you're just starting out.
Figure out the style of bag you want: shoulder bag, backpack, sling, etc. and whether you'd prefer a dedicated bag or throwing an insert into a bag you already own. The 1100D+battery grip isn't particularly large, so if you plan to upgrade bodies in the future, make sure you don't get something that fits your current gear too tightly. If you end up moving up to a 7DmkII+grip, you might wish you had a little more room.
Also figure out if you plan on carrying the camera with a lens attached or not. If I put my 40/2.8 STM pancake on my 5DMkII, it can fit in one of the lens slots. If I put my 24-105 on it, not so much. Two lenses and a flash will mean at least three lens compartments, so those three-chamber shoulder bags may or may not be enough, depending on whether you plan to stack the lenses on top of each other in one compartment, and how the body compartment is configured.