Try 40. They're still a bit large. What's your resolution and are you encoding it in 1080p or 720p? Might wanna check any blu-ray fansub files you have and see their fonts.
It depends on what font you use, too. Some fonts look huge at 40, and others look normal to small. 35-40 should work well, but try watching from 6-8 feet (or 2m) away and see how it looks for an extended period of time. Then adjust it up a point or two to compensate for people with less than perfect vision. Random pet peeve, but I think too many fansub stylists base their "readability" assessment on the subs as viewed while sitting at a monitor staring intently at Aegisub, rather than normal viewing conditions.
If you want moar advice on styling, here's what I would suggest:
* A simple sans-serif font, nothing too fancy.
* White* text with black outlines + black shadows, with shadows set to 50% transparency. Outlines should be completely opaque. Please no "white with blue outlines to fit with the anime" -- I remember watching the Koi TV-fansubs that were styled like that, and several times being in a state of "crying so hard I couldn't read the subtitles."
* Vertical margin of 30-40, or at least something greater than the default 10 that most fansubs seem to use. If the subs are higher in the image, people don't have to keep "bouncing" their vision up and down to read subs at the very bottom of the screen, and can take in the visuals more efficiently. Some would say "well that blocks the image with subtitles," but subs are going to cover up some of the artwork no matter what. May as well avoid gluing people's eyes to the bottom of the screen. Besides, the human eye can interpolate areas that get obscured by subtitles.
*Yellow works too, but I know that people haet it...
Those [FMP Blu-Ray encodes] used R1 subs and are probably dual audio
That's not surprising, considering that you'd have to OCR hardsubs or transcribe+retime to get fansub scripts from Fumoffu or TSR. (Maybe KAA's subs, but idk if they used fansubs or R1 subs.) Either one is a damn tedious process; I just OCRed 2 hardsubbed files, while 9 episodes of another show played on my TV.