I recommend nothing higher than 720p, maybe 1080p in exceptional cases where you like the content very much and you expect integer scaling (up or down) to do a good enough work on whatever player will be playing. 480p can be fine for some things, even 360p (which has the advantage of integer scaling readily to 720p but the effect is only worth it for something like older animated programmes). Audio is different and lighter and you can go with higher qualities (on the same number of channels) without much issue. Remember the basic maths: halving the target resolution and scaling down the bitrate accordingly effectively opens up a reduction in size of up to 75% (each frame saves one rectangle instead of four), but beyond that it really depends on the codec and the visual structure of the content), and the result can be replayed on the original resolution simply by doing hardware integer scaling. As for codecs, if you are already going with Jellyfin, just pick the most efficient codec you can find. If you need compatibility just go with the classics that play even on a 2000s “smart” TV.