Hex screw size - HELP!

[FIGJAM]

I'm fairly new to pc building, so excuse the terribly newbie question.
What size are hex screws on a pc? I'm trying to remove the input output panel on my Antec 900 case, and none of the screwdrivers or allen keys that I have will fit, and they're not the easiest things to measure!

I've tracked down this at Maplin, will it contain the right screwdriver (and any other tools that I might have overlooked?)
http://www.maplin.co.uk/Module.aspx?TabID=1&ModuleNo=33778&doy=20m7

Thanks!

 

[ddwaldon]

If it is a hex head inverted like an allen head or like a regular 6 sided
bolt?

If it is an allen head its probably a 3 or 4mm.
If it is a regular style bolt maybe.
8mm or 1/4 inch.