Hi there - Girls seem to mature at approx 10-12 months and then get a bit plumper with age, boys reach full adult size at around 12 months and then butch up until about 14months.
You are obviously hugely interested in piggies as I often see your posts asking questions - there are some brilliant books that I can recommend. Anything by Peter Gurney is usually worth reading, though my fav is Guinea Piglopaedia. A lot of the information you are after is already in the sticky threads at the beginning of the different sections - well worth reading.
Suzy