When you carry a diaper bag it is for more than just diapers. I have carried diapers, food, clothing, toys, and even my wallet in the diaper bag. So they have to be made with lots of room, lots of pockets and easy access for the mother, or whoever may be using the bag at the time.
I have found a great dimension for a diaper bag is around 20 inches long, 20 inches deep, and a good 6 inches wide. It also needs to have long straps on either side that will allow the carrier to hang it over the shoulder with comfort. These straps also need to be between 1 and 2 inches wide so that they will not cut into the shoulder of the carrier.
The bottom of the bag should be solid, so cutting a nice sturdy piece of cardboard to fit will work fine for this. You can glue some material that matches the bag on the cardboard if you want it to look nice, you can also make a cloth sleeve for the cardboard so that if it happens to get wet it can be replaced with ease. You should also use a material that can be easily washed or wiped off, because the bag will get dirt on it and stuff in it. You should also use a material that is sturdy and durable; remember you are going to be carrying this bag everywhere.
There should be several pockets to the bag. There should be at least two large pockets and several smaller pockets. You will need the larger pockets for the diapers and for extra clothing. There should be enough room for at least two changes of clothing and several diaper changes. There should be pockets for wipes, some small toys to keep baby occupied, salves or creams, and a towel or pad to lay the baby one while changing him, you will also need room for some food, bottles, and if you use it formula (which now come in some great little single-serving packs.) Make sure that there are pockets on the inside as well as the outside. You will want things like small toys easily accessible in case you need them pretty fast, and some things you want to make sure they are close to the diapers so that they are easily found.
One great thing to have is a small cooler type bag that you can put in the diaper bag to carry cold bottles or food. You can usually find these in the baby section of your local everything store. I liked it when I could find a bag that had a pocket that I could use for my own personal items. I hated carrying a bag and a purse around everywhere so I carried a small clutch or wallet that fit in one pocket.
Just remember bigger is better when it comes to a diaper bag but you can get to big. Mom has a bunch to carry and needs to be able to organize things (sometimes for a whole day.) Making a diaper bag organized and easy to get in and out of is the key.Just remember