26" --the mountain bike size, not the English 3-speed size (26 x 1 3/8", also known as 650a; there are actually five different rim sizes that are all called "twenty six inch") is a good wheel size for a relatively small person, but I don't know what frame I'd recommend with that. Possibly an older mountain bike with no suspension, but they tend to be built long, with a high bottom bracket, so they tend to fit like a larger frame.
I have seen older bikes made for 24" wheels that actually fit 26" wheels quite nicely; but putting one of those together might be quite a project.
Could you tell me more about those five 26" sizes? I really just don't hear a lot about them, so anything you'd like to share would be appreciated.
And the longness was a big part of why I nixed building off of an '80s-90s mountain bike frame--I might get the standover height, but reach never looks comfortable for the riding I'd like to do. And I like projects plenty, but finding a dependable 24" vintage frame I like and then building it up with 26" wheels sounds like it might border on an ordeal.