[sampo]

There is Settings / Appearance / Fonts / Custom DPI setting, which takes care of most of things. Panel width is also set in pixels, so you also need to widen the panels.

Firefox does not obey the above DPI setting, so also set about:config / layout.css.devPixelsPerPx separately. Yet this doesn't seem to affect video controls in YouTube, so they are kind of tiny.

Finally, nothing above scales the window title bars. I kind of like the title bars thinner, because they are wasted space anyway. But if you wanted to increase their height, apparently you'd need to edit the png files in the window theme files, because Xfce does not attempt to scale them, but uses them as pixel graphics.