Baby Nappy

How to choose a baby bag/nappy bag?
Just for when your out and about and need the basics.
Im a first time mum to be and I want something not too bulky but not with unneccesary room.
Anyone give me any tips on choosing a good one?
how much room does there need? do you need compartments etc?
its really hard to tell someone how to pick out a purse. What you need to do is have your baby, THEN go out shopping for a bag. For some people all they need is one little pocket and a bottle pocket, and others need like 10 different compartments.
baby nappy change