How do I know what size to choose when knitting or crocheting baby clothes?

You've got your pattern and your wool, you've settled yourself down with your needles and a cup of tea. You pick up your pattern and the sizes are in inches and centimetres. Without a baby or small child in front of you to measure, how do you know what size to make?

This is not a definitive sizing guide but I have looked at patterns from a number of different patterns manufacturers and come up with an average. 

I hope it helps