ffmpeg hardcoded subtitles from the top, bigger, with thicker stroke, on a dark background
Aegisub

Use ASS subtitles. Style them in Aegisub. You can style each letter or word individually, or apply the desired styling to all subtitles. See the documentation for detailed instructions.



Example ASS file:

[Script Info]
; Script generated by Aegisub 3.2.2
; http://www.aegisub.org/
Title: Default Aegisub file
ScriptType: v4.00+
WrapStyle: 0
ScaledBorderAndShadow: yes
YCbCr Matrix: None

[Aegisub Project Garbage]
Last Style Storage: Default

[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,Arial,48,&amp;H00FFFFFF,&amp;H000000FF,&amp;H00000000,&amp;H80000000,0,0,0,0,100,100,0,0,1,6,0,8,10,10,20,1

[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.00,0:00:05.00,Default,,0,0,0,,Top of canvas. Bigger font size. Thicker stroke.


Without Aegisub

You can apply styling via the subtitles filter force_style option in ffmpeg. This should work with any format that the subtitles filter accepts.

ffmpeg -i input -vf "subtitles=subs.srt:force_style='Alignment=6,Fontsize=48,Outline=8'" output


Refer to the example ASS file for accepted force_style options, but it will not allow you to style each letter or word individually as you can in Aegisub.

Note that ScaleX and ScaleY values when used in force_style are a factor, not a percentage, so the accurate values would be 1,1 (not 100,100) in that case.

Transparent background

This can be tricky due to some limitations of libass, but it is possible. See How to set background to subtitle in ffmpeg?