Open KDE settings -> appearance -> fonts and note the size of the general font.
Open Better inline clock settings and set the font to the same size:
Apply. Result:
The result is that the clock font is much smaller than it should be.
This is a result of interpreting the specified size as pixels instead of points. On a 96dpi monitor there are 96 pixels per 72 points. If you desire the clock to use the system default size of 10pt, it would be necessary to enter a pixel size of 13.3333333333 pixels. However, because the input field only accepts integers, this is not possible. Therefore it is impossible to have the clock use the system default font size.
To reproduce:
The result is that the clock font is much smaller than it should be.
This is a result of interpreting the specified size as pixels instead of points. On a 96dpi monitor there are 96 pixels per 72 points. If you desire the clock to use the system default size of 10pt, it would be necessary to enter a pixel size of 13.3333333333 pixels. However, because the input field only accepts integers, this is not possible. Therefore it is impossible to have the clock use the system default font size.