Hi and welcome to Knitting Help!
Measure at the lower edge of the sweater not the neck area. This is going to be the widest part of the sweater as the short rows are used to shape the yoke and neck area.
This sweater is knit in one piece so you'll knit the back after you finish the first sleeve.
I don't know if you've seen the "5-hour baby sweater" but this is an earlier version of the same pattern. It's a lovely pattern and lots of fun to knit. Nice too that it has the bonnet and booties.