Doesn't work properly in IE. IE doesn't support some of the HTML5 features necessary to run this app, and I don't have the energy to hack together workarounds. Sorry, IE users.
Not sure how well it handles mobile/forms of input that aren't mouse-drags. Testing and possibly remedies to come.
Doesn't handle very large images very well. In progress.
Saved image doesn't include the labeled table. In progress.
How do I use this?
Load an image.
Draw your baseline - this is the line that determines how much to scale all the other calculations. Height or chest-width are good choices on full-body shots, while palm-width can be good for close-ups.
Enter how long this element should be in real life (for instance, your height/chest-width/palm-width, or that of the person you're trying to scale things to). Units doesn't matter - the math works out the same no matter what, and whatever length unit you put in (inches, cm, etc) is what you'll get back.
Start drawing more lines for stuff you want the dimensions on.
When you're done, click save to download a copy of the image with your lines and measurements on it!
I only have really tiny images!
You can try resizing it with a smart rescaler like waifu2x (work-safe, despite the unfortunate name). While I'd like to provide a version of that here, it requires better hardware than my webserver has, unfortunately.
I closed my browser/tab and everything went away!