You could go to More tab, Beta Features, Large Print Mode On.
it makes all fonts in the UI larger. It's far from ideal, since it affects a lot more than just the dictionary. However, it is an option.
I see that there's also a patch that looks as if it might be relevant:
Dictionary text font-family/font-size/line-height
I've never used it myself, so I'm not sure exactly what it does or how it works. Perhaps it's worth playing with it? It's easy enough to remove a patch if you find that you don't like it.