Here I read the following:
Now I wonder: how big can the virtual screen size be when using radeonhd (or xf86-video-radeonhd) without losing DRI? Or isn't it a driver limitation, but rather a limitation of the grapich card?Note that the maximum supported size of the virtual desktop for the Intel 945GM series of chipset with 3D acceleration enabled, is 2048x2048. The virtual screen can be larger but DRI will be disabled.
Edit: Woops. I wonder if DRI is available at all in radeonhd..