[Calculator] Fix popup size and minimum size for expanded version

Summary:
In D21503, I messed up the popup size and made it iconify at too small a size. This
patch aims to fix that.

Test Plan:
I'm not super familiar with this code so I'm not sure it's the right approach, but
it does seem to work:

Reviewers: davidedmundson, Plasma

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21811