I hesitated making this an answer rather a comment, as it is more of a workaround and there's room for improvement in Plantuml. Ideally diagrams shouldn't need manual tweaking, although it would be a rather wide diagram if the text length was honoured. There may be a MaximumWidth style to force word-wrap, but I couldn't find it.
But for the first one, you can use newlines to separate the two lines of text:
Also, it may help to know that the space available to the label text is dependent on the size of the action box below:
For the circle one, you can use a root level margin. But I don't know why the circle is off-centre.