Say you have a recording created with
Asciinema named recording.cast and now you want to convert that into a GIF. You can use the agg command (which is a
separate installation) to convert it like so:
agg recording.cast recording.gif
However, if you run agg with the default settings the resulting GIF won’t be high quality and it’ll look a little fuzzy.
To get a high quality GIF you need to specify a big font size with the --font-size option:
agg --font-size 64 recording.cast recording.gif
Here are two GIFs you can compare. The top one was created with the default agg settings. The bottom one was created with --font-size 64. Depending on your display, you may need to zoom in a bit to see the difference:

It’s a noticeable difference. The trade-off here is that the higher quality GIF will be a bigger filesize, so be mindful of that. Try experimenting with different font sizes.
I was expecting there to be some sort of --size or --quality option, but you just need to increase the font to get higher quality GIFs with agg.