MFT size in XP windows

The command in question only alters the space reserved for the MFT, it does not change the size of the MFT and it does not reduce free disk space.  This is a soft reservation. If space is needed for normal files it will be taken from the MFT reservation if needed. If the MFT needs more than the reserved space it will be taken from disk free space. Under normal use this will be unlikely. 

For normal usage the default MFT configuration will be best.  That is why it was chosen as the default. The exact size reserved is dependent on a number of factors and is undocumented. Setting larger numbers in the command in question would normally serve no useful purpose. This would only be of use if you had an unusually large percentage of very small files.

I don't know of any way to manually enlarge the MFT and see little reason to do so.

See this article for more information on the MFT:

http://support.microsoft.com/kb/174619

A small number of MFT fragments is nothing to be concerned about.

The file system is more than capable of increasing the size of the MFT when this is needed. There really is no difference between doing this yourself (if it were possible) and letting the file system do it. 12.5% of the disk space might be reserved for the MFT but that doesn't mean that the MFT is actualy going to be that large. The MFT will only grow to the actual size it needs to be so there is really nothing wrong.

I see no reason for concern.