The upper limit is otherwise hard drive space. That is, if you want to use 16384x16384, do so, but no video card nowadays supports anything that high. That would require 3/4 of a gigabyte in video memory at 24-bit color. Such sizes will only be available about 2 or 3 years from now. The highest I've seen for video card support is 4096x4096. According to Wikipedia, there are several variants of the GeForce 4 series. It also seems like a fairly old video card, from early 2002. The best option is to either upgrade your video card (Radeon 9600 XT is available for $80, but I need to know your motherboard's specifications first) or create mipmaps for your texture. Textures don't use nexus, but the entities, vertices, faces, and other objects do.