I'm going to preface this with - I am not one of those organised mothers who is prepared for everything...rarely even truely prepared for anything.

So with that size is a big thing for me. I'm a wallet and keys girl, I don't carry my own bag, let alone a suitcase for everyone else. I wanted something that held the essentials but wasn't heavy as lead when full.
I have had several now and am currently using an old handbag that hardly saw the light of day. It's a better size.

I found that hand bags with pockets annoyed me because stuff always ended up jumbled anyways so I preferred a nappy bag without too many.

Think about how you tote your stuff round now and that will give you an idea of what you need.

What suited me was a big bag in the car and a little bag to sling in the pram - so I technically had what I needed, but I just didn't have to lug it around.

Sorry that was a bit of a ramble ....