What is the maximum LUN Size supportable in Window 2008

Windows 2008R2 (64-bit) will support Petabyte capacity LUNs.  The 32-bit version will support 3 TB.

My Windows version is 2008 X64 Standard Version SP2

You are good to go.

Created a LUN of size 2.5 TB and assigned the same, but when i am scanning the disk, automatically it is into 2 disk

one of 2 GB and second is of .5 GB. So I have created 2 disk one of 1300GB and one of 1200GB. But anyway want

to know why it was happening and windows is not taking more than 2TB of LUN Size.

Convert to GPT

When you initialized it you likely took the default of MBR versus the GPT partitioning scheme when prompted (or less likely you have GPT partitions but only have a cluster size of 512 bytes).  The following Microsoft KB article is a good reference:

http://support.microsoft.com/kb/302873

As mentioned in the article, depending on the cluster size that it is formatted with, it can be up to 256TB (64K cluster size), but as you'll read only up to 16TB has been "tested".  Despite 256TB being suggested as the maximum, it would be the scheme used to take it even further (petabytes, exabytes, etc).  Also, when selecting the GPT partitioning scheme, alignment to the sector boundaries of the CLARiiON disks is still recommended, but the process is slightly different as you now have an MSR (Microsoft Reserved) Partition to deal with and changes the diskpart commands as documented in the following EMC KB article:

Then again, as you have done, you worked around it by carving it up into two partions less than 2TB each.

Actually just reread and seeing that the op specifically asked about Windows 2008, even GPT partitions are aligned by default with a multiple of 64KB (as is the case with the MBR partitioning scheme) in Windows 2008.  So the EMC KB article: emc209704: "How to align a Windows GPT Disk" should only be necessary with Windows 2003 SP1+ when GPT partitions were first introduced and not Windows 2008 GPT partitions.

Then again, only takes a few seconds to run the diskpart command (list partition) to verify alignment.