what is the effective size of the server if i have 4 250GB disk?
is it 0.5TB or 1TB?
The short answer is "It depends". Specifically, it makes a difference what folders you enable duplication on. Worst case, your 1 TB of physical disk space will give you 500 GB of "logical storage". Best case: 1 TB.
For each folder (both predefined and user created) you have the option of specifying if the contents of that folder should be duplicated on another drive.
On my home machine, I have ripped music and photos folders set to duplicate. I've chosen not to duplicate my ripped DVD folder, as DVDs are quite large and I can alway re-rip.
In short, this isn't RAID. WHS has both advantages and disadvantages relative to RAID. One of the nice advantages of WHS is that you can add disks incrementally without requiring all disks to be the same size.