Once upon a time... I was toying with Lancelot launcher's resize handles and after getting it to the smallest size possible, I found that its vertical size was refusing to go back to normal. Deleting the widget and adding a new one showed that the size is resilient.

So, the question is: Where is the relevant configuration file stored ?
(grepping the output of "ls -R" in my home dir for 'lancelot' did not yield any result)
Also, can someone else confirm this behaviour ?

The Lancelot configuration might be stored in ~/.kde4/share/config/lancelotrc
You may need to substitute ~/.kde4 for ~/.kde if needed.

Otherwise, try removing Lancelot and adding it back in again.