i want to know what size of hex key i need to fix my tremolo, because i can't play that good with this problem.
if someone knows, let me know because i'm tired of playing like that.

If you are talking about adjusting string height with the hex head screws on the saddles MIM is 1.5mm (metric) allen wrench and American strats use .050 std. If your setting your tremolo then click second link...
http://www.stewmac.com/shop/Tools/Wrenc ... _Keys.html
http://www.fender.com/support/stratocaster.php

_________________
An object at rest tends to stay at rest...
and an object in motion tends to stay in motion

If you look under the Support section of this website, there are some very good setup guides that have a great deal of this information (and there's lot more on the internet if you just Google "Strat setup".

That said for the hex heads, I would suggest going to your local hardware or tool supply store and picking yourself up a set of the nice multi-hex wrenches (has several different sizes built in to the handle). If your instrument is an American or even an MIJ, then a Standard set of wrenches is probably all you'll need but if it's an MIM or other import, then you may very likely need both Standard and Metric (I know the bridge saddles on my MIM's are metric but the truss rods are actually standard so I need to have both).

Peace,
Jim