The driver auto-detects all device info included memory size, so use the following device section in your XF86Config file:
Section "Device" Identifier "NeoMagic" EndSectionor let xf86config or XF86Setup do this for you.
But if you have problems with auto-detection, you can specify:
VideoRam - in kilobytes
DacSpeed - in MHz
MemBase - physical address of the linear framebuffer
MMIOBase - physical address of the memory mapped IO registers