Fonts
Ethiopian languages, like Gəʿəz, Tigre, Tigrinya, Amharic, are typeset using the AbyssinicaSIL font which has the widest support for Ethiopian languages at the time of this writing (August, 2021).
Font sizes correlated with “Brill” font sizes
AbyssinicalSIL: 9.35 pt ~ Brill 11 pt
AbyssinicalSIL: 8.5 pt ~ Brill 10 pt
AbyssinicalSIL: 7.65 pt ~ Brill 9 pt
Source of font
The Abyssinica SIL font is distributed under the SIL Open Font License, so its use (among other things) is free.
In HTML
The type size of AbyssinicaSIL text should be 83% of the size of surrounding Brill typeface text.
Windows and OS X
Windows fonts, as long as they are TrueType fonts (.ttf) or TrueType Collections (.ttc), or CFF-flavoured OpenType fonts (.otf) can also be used on OS X.