Hey, i am expanding storage for a dell 2850 server which runs a xeon dual core.
Does anyone know the maximum possible size i can extend a logical volume to? ive read somewhere that for typical x86 architectures kernel support only goes up to 2 terabytes...but i need about 15 terabytes

The filesystem imposes the biggest practical restriction on logical volume size. For example, the limit for ext2/3 would be 4TB using 4K blocks. At 15TB, you would need to use something like ReiserFS for a single logical volume, but you can always use multiple smaller logical volumes for ext2/3.

If you are using LVM1, the maximum volume group size is limited by the selected physical extent size, much like the maximum filesystem size is limited by the block size.

You may have been reading about the 2TB per block device restriction of the 2.4 kernel.