Regarding build, the wiki has the details: Attributes (NPCs) - MoR-Wiki
Specifically, female NPCs can go up one body type from their "genetic" base (so a Petite can become Normal, a Curvy can become Plump, etc.), and if they've already gone up one, they can go back down to their base. No other build changes are possible currently.
This is determined entirely by weight. The numbers are in the wiki section I linked. If you have your options set to metric it's just their weight in kg, otherwise you would need to convert from pounds to kg. You can increase weight by making sure they end the day over-fed (either by setting their rations to 5R per day, or by leaving them at 3R and taking them to diner every day). A high strength indirectly contributes by automatically increasing their weight up to a certain point. The calculations for that are somewhere else on the wiki but personally I find it more reliable to just overfeed them. Just note that it's only a chance per day to increase their weight, and it can take a bit of time to build up their weight to the next tier, especially if you're trying to go from Curvy/Tough to Plump/Muscular since you need to get them up to 85kg