Thanks for all the suggestions!
I went with a slight variant of EricShulman’s solution. The line-height by default seems to be quite a bit larger than 1em, something like 1.4em, so it looked bad when I used 1em. I ended up doing line-height:1.4em which works well.
Unfortunately CodaCoder’s suggestion did not work for me, though I think it would work for the latest version of TiddlyWiki. I’m using a slightly older version; I tried to upgrade to latest, but it broke some other stuff in my TiddlyWiki that I need (namely MathJax support for latex).
It still would be great if one could do in an in-app zoom in TiddlyDesktop (TW_Tones suggestion does not work for me, though it does work in say Firefox; unfortunately it seems one can no longer edits TiddlyWikis with firefox). One advantage of this would be that I could easily use a different level of zoom on my laptop vs desktop.