Incr. size dummy QACanvas to 5x5 pixels, fix bad text size w/ Deb12

Redmine: Refs #3151

Avoid crazy values in automatic computation of Pad and text sizes under Debian12 (and maybe other OSes, but at least on this one it leads to an error message)

@f.uhlig not sure if it needs to be backported to the release as only the tracking QA is affected from what I could see in the CI