The Wii U Controller has a LCD. But Nintendo didn't provide specs on the Wii U Controller's display, such as pixel count and resolution. So people can only judge with their eyes.
That's exactly what website Golgotron did. It's a bit of backyard science—okay, it's a lot of backyard science—but the site manually counted pixels on hi-res photos, coming up with 870 pixels across.
"However, that doesn't jive with any standard screen resolutions and it certainly doesn't match any LCDs commonly in production," wrote the site. "854, though...now that's a winner. 854 x 480 (WVGA) is a very common widescreen resolution, and it's within 2% of our not-scientific-enough measurement."
Nintendo Wii U Tablet Screen Resolution Revealed [Golgotron.com]