I would say probably 1280x720 or less for the best chances at getting your game to run smooth on phone & tablet devices.
1080p may be possible, but you may have a hard time due to required storage space & required system resources it will be using.
You can optimize your games further by using webp image format from the very beginning instead of png. Webp is really great at reducing down the total required storage space for your game as a webp lossless image file will probably be somewhere around 25-50% smaller than the original png file, & webp lossy @ 100% quality will be about 50-75% smaller than the original png file. All in all, much smaller storage space required, much faster loading times for images & animations which is good for phones & potato pc/laptops, however the VRAM usage, etc won't be affected by the file sizes.
Anyway here's the link to the image encoding page I wrote on the wiki, it also includes a link to xnconvert & a guide on how to set it up.
https://wiki.visionaire-tracker.net/wiki/Image_EncodingQuick note: everything I've said here is mostly speculation, but it's what most of us have agreed on when we've discussed it on our discord server as it's a nice inbetween resolution that still looks decent on larger screens.