My kids are learning at the moment . age 11 and 14 . Girls approx 40kgs. They using a twin tip 120 by 36 using a cab switch 5m and a Ozone Uno 3m . That is going fine to learn on .. They not really getting going yet but it's just baby steps ..
You could go bigger though .. I reckon you could go between 122 and 130 .. One of the shop guys even suggest that I try putting the kids bindings on my 140 if they are having trouble getting going .. Just to give them a bit more surface area and pop out the water a bit more. ..Haven't tried it yet put prob will this summer.
Damn I wish I started out when I was 13 rather than 40 ..