Text displayed un capability diagram

This is actually a Sirius limitation (Sirius is the graphical DSL on top of which Capella is built).
For I don’t know which reason, node labels when they are on the side cannot be resized and use multiple lines, while connector labels can. There is no logic here.
You can file a defect on Capella (we will then see with the Sirius guys what is planned about that).

for information, concerning the line return i have a trick, i write the text with notepad for example (including line return) and i copy paste the text into capability name inside the diagram.

I just tried and indeed, the tricks works, e.g. Capability name = “My [Return] Capability”. Note that when checking the property view, name becomes “MyCapability”, all attached.