Today I added the soft constraints for line length and arc radius. There are still some small problems for the line tangent to an arc constraint, but the resize visible in the original video is not quite as bad. I'll continue working on this and add the arc angle soft constraint.
As the problem was most obvious with concentric arcs, I recorded a video that shows the concentric arcs constraint applied without and with the arc radius soft constraint. All soft constraints will be applied automatically; if adding a soft constraint causes the sketch to be over defined, it will not be used.