I started a new part, installed an image of an object (a scan) in a sketch, and traced around it using spline curves. I also included a ruler in the scan, and draw a rectangle around part of it. I hoped to scale the spline and the rectangle until the dimensions come out right, so I'd have a full-size outline to extrude from.
The rectangle scales with no problem, but the spline always fails with an error about constraints and it not being scalable. It was made from 3 parts, but connected, and I can't see any constraints except connecting the ends. Any suggestions?
Why didn't you get the image to 1:1 scale in your sketch first, that is what I do.
Try converting your spline to a Sketch Block, you should then be able to scale.
Attach your file here if you can't figure it out.