better question is:
do you have a SSD or HDD?
if SSD yes manually set it to one size, you don't want constant resizing of the paging files on your SSD. It will cut down the lieftime of your SSD. Better yet move it off the SSD and ono a regular HDD.
If HDD i would say let windows manage it BUT if you had a secondary drive i would move the paging file onto the drive without the programs you use the most. For example if you install Firefox and excel and word and WoW and BF2 on drive C

boot) then move your paging file to dive D

if that is the secondary drive) Be careful doing it on Partitioned drives (500gb into 250gb x2) as there will be no difference in drive access times.
Hope this helps.