Just to add another datapoint, I've been working on adding more content to The Shipwreck Arcana, and as part of that I needed to add more card images; I landed on 250x427 (mostly because the source assets that I have were double that in each dimension). That seems pretty similar to what VictoriaLa mentioned!
They look much nicer and the text on them is way more readable than in the currently-published version, at least to me. Even on mobile, I'd expect the final assets to be cached after the first time that they load; I'm probably not going to be redownloading them over LTE while I'm running around! Here are pictures of similar assets "before" and "after":
https://i.imgur.com/c7JURCi.png,
https://i.imgur.com/tylxkEw.png. Other quality stuff aside, the cards in the "before" assets are 100x256.
From the perspective of also being a player of a lot of these games, I agree with what everyone else is saying---please consider using a little bit of extra storage and bandwidth to make the game look nicer!
If preprocessing, scaling, or spritesheeting are giving you trouble (or if there's anything else that I can do to help), I'd be happy to share what my asset pipelines look like. They aren't fancy (they're a combination of shell scripts wrapping ImageMagick and Grunt (the JavaScript tool) build files) but they get the job done pretty effectively, and in a repeatable way.