I read in this site that complete X-loader is quite big for onchip SRAM of beagleboard-xM, so its divided into MLO and uboot, and MLO configures DDR then uboot is loaded into DDR. When is SRAM configured then? What is the actual size of SRAM and DDR in beagleboard-xM? where actually are they in it?

sram is pre configured by default, you don't have the access to it's
parameters. Ddr is configured by MLO (x-loader)