Interesting question. On the one hand one is limited due to image quality but on the other hand buyers probably search larger resolutions for some special content.
I currently go for 4092 x 2340 pixels. I also have some images with 5376 x 3072 px. This is equal to 2 or 3 x upscale.
I don't think it makes much sense to go further if you sell content which is mostly buyed for mobile usage with smaller image resolutions. I mean even for website banners you would buy smaller sizes.
You need really huge resolutions only for printing.