From the pattern:

Gauge is 13sts and 28 rows to 5cm over garter st.

US Number 3 Needles/2 ply yarn.

Original pattern was knit on size 3 needles, of 3 ply baby yarn and looks like it may fit new-born to three months. To make it bigger, just use appropriate yarn you want (such as knitting worsted) and then use the suggested needles and gauge for THAT yarn. Knitting worsted on US size 7 comes out bigger - maybe 6 to 9 months size.

This is an easier sweater to knit than it may look like. I will be happy to answer any questions you may have when making it and would love to see more examples of ones that have been made.