The height of the rows 7,..,23 is in a certain Excel sheet to low.

How can I increase the height (for only these rows) by 8 pixels ?

Or more general: If I mark rows n,....,m
How can I make them x pixels (resp y millimeter) higher ?

Try this tiny macro:

Sub row_your_boat()
For i = 7 To 23
Rows(i).RowHeight = Rows(i).RowHeight + 8
Next
End Sub

To be clear for the OP, your subroutine increases the RowHeight by 8 points,
not 8 pixels.

I do notice a slight discrepancy (not your fault, the system's font handler
is doing it) in that the default standard height for Arial - 10 Points is
12.75 points high; adding 8 to that should make the rows 20.75 points high,
but my system shows it becoming 20.25 points high instead.

Rick