[plasma-pa] Increase minimum size of expanded plasmoid

Summary:
When outside of the tray, the plasmoid felt really cramped. This makes it more bearable.

Before: {F6615039}
After: {F6615040}

Reviewers: #plasma, broulik, drosca, ngraham

Reviewed By: ngraham

Subscribers: abetts, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19013