Salaam
I assume you've got the arabic script either through the 'character map' or an internet keyboard, in which case Microsoft Word does not fully recognise it like a normal font... well, not as far as I know
The solutions suggested above will actually install the arabic font onto your computer, thus enabling Microsoft Word to treat the characters just like any other font ... so that you can increase the font size, reduce it, make it bold, apply a shadow etc.
I know the procedure might seem long-winded, but it really is worth the result.
Fee amaanillah
Wassalaam