One of the most common questions I get when I post a new doll or figure without any context for their height is about the relative sizes of the figures. I'm hopeful that this comparison will help give some context to the discussion.
Obviously this doesn't include dolls/figures from every line out there (or even every line I own), so if you're curious about something specific, let me know and I'll see if I can take some additional photos!
Ellowye Wild, full sized American Girl, and MSD/mini-range BJDs are 18"/45cm (the Souldoll is 52cm, and is a non-standard size). Adult-shaped dolls in this range are 1/4 scale, while proportionally the AGs (and similar playline dolls) are somewhere in between 1/3 and 1/4th as they're meant to represent children.
The Sideshow and Hot Toys figures as well as Monster High and Barbie dolls are 12"/30cm and are generally 1/6 scale.
Square Enix/PlayArts and some older figures from Playmates are 8-9"/ 22-25cm and depending on the proportions are between 1/8 and 1/10 scale.
Figma are 1/12 scale.
The Marvel and Star wars figures shown here at 1/18 scale, which is also called "Dollhouse scale".
The HeroClix figure is just 3cm in height and is O scale (model railroad scale, between 1/43 and 1/45).