The foremost question I get asked about my baby bootie patterns has to do with sizes.
The biggest problem that arises from designing your own sewing patterns is sizing.
As for my baby bootie patterns, I always encourage buyers to measure their infants foot to determine the size to make.
I size by inches, Not by age.
The reason I size by length is there is no set standard. There really can't be when sizing by age. This causes everyone who makes baby patterns to do their own sizing, even tho we try to make them close to the same sizes. We typically use shoe size charts for booties. Another reason is because most booties are sized by Months... ie. 0-3 months or 3-6 months, etc. Typically 4 inches = up to 9 months. 4.5 inches = up to 12 months. Your infant could have been born with feet under 2 inches to over 3 inches and by the time they are 3 months old, they could be 5 inches, depending on if your Newborn baby is a Preemie or a 12 pound Toddler and how fast they grow.
Measure, Measure, Measure! Check the finished length of the bootie to your infant's foot measurement and allow for wiggle room and/or growth.
Here is a picture of baby shoe forms (lasts). They are marked as being the same size, but there is over 1/3 inch difference between them (1 whole size). I used the yard stick to make sure they were even in the back.
My current sizing chart...
Large = 4 inches.
This chart will also change a bit, when I get my new patterns completed
and when I update my older patterns and add more sizes.
This chart is what I base my sizing on. I don't use 1/2 sizes,
and I base whole sizes on 1/3 inch increments. So... if your foot measurement is 3 inches (size 0), you would want a bootie at least one size larger (size 1).
Don't be daunted! It is very rewarding making baby booties for your own babies or for your friend's.