Scales, Munson plan now.
Lol just kidding... well sort of
1.3 ft is a good size as far as i can tell for a 5 month old, I'm a bit of a newbie to and I follow the munson plan religiously. My corn powder is just over a ft, about 18g in weight and I estimate that shes a late 08. I moved her up to double pinks about 2 weeks ago and shes doing great, shes around 8-10mm at her widest point.
Hope this helps.