Comment 2 for bug 1604227

Experimenting a bit...

First, the high DPI did not seem to do anything, merely by clicking the "Enable Support..."
box, and leaving the "Scale->Auto" box checked.

I noted under Interface->Style there was a GTK+ option, and things looked a bit bigger and better than when Fusion (default) was in effect - the menu items were somewhat larger, also.
Strangly, when I switched back to Fusion to clarify the difference(s), the size remained the same.
Also, when re-started SMPlayer after selecting GTK+, the menubar was back to being miniscule.

I noted that Default Font was blank. Also that when I clicked on Change, the selected font displayed was Ubuntu Medium 14 (although it had selected Medium Italic, for some reason...
I modified it to just Medium.
Nothing changed on the menubar until I restarted SMPlayer. The menu font was larger. Not as large as I would like, but that is obviously changeable.
I went back to HighDPI, unchecked Scale->Auto, and put the slider all the way to the right ("4"). After restart, everything was HUGE, except for the font size, which had not changed at all...(!) Why would the font not also scale up? I put it back to Scale->Auto.

I don't know why the font selector for SMPlayer knows what the system font/size is (except it is not italic), yet it does not default to that font automatically, and leaves the font box blank. Whether this is a bug or intentionally odd, I don't know, but in my humble opinion,
SMPlayer ought to default to the system font/size, and place that info in the (currently blank) Default Font box in the Interface section of the Preferences.