I's like to ask if there is a way in Plasma to create a widget that automaticaly adujusts it's size to it's content.

Like a widget with a list, that becomes bigger if the list gets new elements, instead of displaying a scrollbar?

Plasma widgets are not allowed to resize themselves as far as I am aware, although they can specify a size hint, and a minimum size ( since this is what standard Qt widgets can do anyway... ). Changing the minimum size on the fly to achieve this effect would however not be a good idea, as Plasma's behaviour is undefined.

actually a widget could call resize(newSize) when it want to, but i'm not sure it would be a good idea, 'cause a user could have other plasmoids that would be covered when it resizes itself.