The following Soles Crochet Chart can be used when making crochet baby shoes or booties. I have used this chart with Clover 4/0 (2.5mm) crochet hook and suitable yarn to create soles of about 9-10cm long – which is suitable for babies between 0-6months old.
If you use different sized hook or yarn, then obviously the sizing will vary. This is but one method or pattern for creating the soles for baby shoes/booties.