my gtx260 will do 8192x8192, but at the 4/8 bpp for dxt1/5 respectively, your talking about a 32/64 meg texture. but thats one texture, with our texture system, you need 4 of them, 2 of which absolutely must have alpha support. so were talking 32 (64 with alpha)+32+64+64, thats 192 (224 with alpha). mind you this does not include mipmaps (which at this resolution would be huge by themselves) considering a video card with a gig of ram (and mine has less than that), this is about 1/5th of the memory on the card. therefore this resolution is not even remotely sane for this type of application. these texture resolutions are mostly reserved for world geometry textures in other types of games. if you want a mission where two super-caps duke it out, youve used up half of your video memory.
i kinda have to agree with the mediavps methodology here. its not about supported maximum texture size but in the amount of video memory on the card. 4096^2 should be the absolute maximum right now, and lower res textures should be made available to those who have hardware that does not support those resolutions. but a good practice is to draw textures at a much higher resolution than supported, and down sample to the maximum and sub-maximum sizes. that way when the average video memory goes up, you can release super high detail textures from existing sources.