Try as I might, I can't adjust the size of a self-synchronous call (first image) in Umbrello (Umbrello UML Modeller Version 2.32.3 - Linux).
With difficulty, I can adjust the size of the time line for a synchronous call from one object to another (second image), using the mouse on a left-click bottom right corner of the block. It's a bit fiddly, and doesn't always work first time. But I can't do this, for a self-synchronous call i.e., when the object calls its own method. Am I doing something wrong?
Self-synchronous call - size can't be adjusted
Synchronous call to another object - size can be adjusted