I exported an image (from R) to SVG and want to modify it with Inkscape. My problem is that when I change the font size, it only changes it vertically, the width is not changing at all, which therefore disforms the text. The width of the text is somehow stuck and I don't know how to fix it.
Sounds like your text element has a
lengthAdjust attribute set.
Open the 'Edit → XML Editor...', select your text object and delete these by clicking the trash icon next to the attribute names. You may have to adjust the font size again for it to take effect.