Formatted ASS subtitle makes it impossible to read in mplayer preview

[1dolla]

(macOS, Jubler 8.0.0, mplayer 1.5-4.2.1 via MacPorts)

Maybe I'm completely missing something here, but I've been trying to set up mplayer properly with Jubler, so I can have some nice previews of what I'm editing.

I'm dealing with 4k and 8k videos, so I use -xy 800 as mplayer argument to avoid the mplayer window taking up the entire screen and blocking the Jubler window.

The subtitles are then absolutely tiny and unreadable. So I try to add arguments to increase the size of the subtitle in the mplayer window, but because the temporary subtitle file that Jubler creates is an ASS file with specific font directives, e.g.:

/var/folders/k5/jb4pjqn14ms8ph8kxb71q2v00000gn/T/jubler_11295766013046402661.ass

[V4+ Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
Style: Default,American Typewriter,14,&H00FFFFFF,&H0000FFFF,&H4B000000,&H4B404040,0,0,0,0,100,100,0,0,1,0,2,2,20,20,20,0

then mplayer ignores font rendering arguments like -subfont-text-scale.

Changing the font size to the max available 72 makes the subs just legible in mplayer for 4k videos, still unreadable for 8k videos. And with the side effect that the Jubler UI is now unusable because the subs are huge.

How do I resolve this? Am I simply blind that I can't find the option to use fractional sizes, or to not use ASS as a temporary format, or to not format the ASS as that's not a requirement, but an option for that format? The sub is loaded and saved as SRT. I've been trying things for hours.

Help :'(