While huuuuuuuge textures are possible, don't go there unless you have a good reason too. As mentioned they increase file size and load time, but more importantly they eat up GPU memory needed for other things. Most things are fine at 1024^2, you could go as high as 4096 for specific things, but at that's mainly necessary when skinning (eg using models). For brushwork using a lower texture scale and multiple textures is the way to achieve high detail. Also note that if you're looking for max texture sexyness, begin with making sure you turn off all compression options:
Code:
gl_texturecompression
gl_texturecompression_2d
gl_texturecompression_color
gl_texturecompression_gloss
gl_texturecompression_glow
gl_texturecompression_lightcubemaps
gll_texturecompression_normal
gl_texturecompression_q3bspdeluxemaps
gl_texturecompression_q3bsplightmaps
gl_texturecompression_reflectmask
gl_texturecompression_sky
gl_texturecompression_sprites
gl_picmip
gl_picmip_other
gl_picmip_sprites
gl_picmip_world
gl_max_size
Set all those to 0 and do a vid_restart. Be warned though, unless you have a gpu with tons of onboard memory, even the default maps and assets will likely mean constant dipping into system memory which in turn means lower performance.