Hi Simon, from my experience and I am sure we are talking straps here, taking height and weight of your gf into consideration, I would say go 133x39. Shame we don't have C's old Jamie anymore. Not too flat, floppy, booring, but not too much rocker either. That other board you are looking at now is slightly wider than what this length normally comes with. So could also work. Depends on whether your gf has large or small feet really (relevant for the ability to edge). 131-133 with 38-40 width should be fine. Should she want to ride in boots at some stage you need to go 135 or so instead probably.
Girls boards is pure Marketing, you can add a pink sticker yourself. And you can pick a board which is not too wide yourself as well.
It is probably too early to tell. But a relevant factor is also whether she prefers riding rather powered or takes it easy and likes it underpowered. Which a lot of girls seem to like, in the beginning at least. In that case the first board should probably not be too small. Especially for your flat water spot where going/staying upwind is important. In heavy chop on the other hand and fairly powered the smaller board comes in handy as you know.
See you on Sunday perhaps