It isn't about "lag" and anyone running a machine without enough VRAM to handle them shouldn't be playing games in the first place.
However past 512x512 you're talking major diminishing returns unless you're running the game at a stupidly high resolution. We're talking 4k+ before it's honestly worth having them, the pixel density just isn't high enough at lower resolutions unless you're zoomed right in.
However the game does support them, infact your GPU will hit it's max texture wall before the game does, so if you want to see what I mean feel free to make some silly high res ones and pop them in your custom folder to test them out.
Ideally ones with a lot of fine 1px details, no point in using Minecraft quality art at that resolution. ;)
eg 16,384x16,384 is technically possible if you have the hardware for it
(that will give you 128x the detail of a 128x128 texture)
Last edited by SkulFuk; Jan 28, 2017 at 06:54 PM.