Res. Size does it really matter?
 

I'm about to construct my watercooling box out of lucite and will be making my resevoir built into the lucite box. The thing is my box dimensions are 12x15x5 and the res. will end up being quite large should i fill it completly with dionized water or only a partial amount of water?

Distilled water, not de-ionized.

The level doesn't really matter but... you want to keep the inlet submerged, otherwise you'll have a splashing sound to contend with.

You do want to make sure that the res is the highest point in your rig.

Post a pic?

Having the reservoir at the highest point generally makes purging air easier, but is not a requirement. And by "inlet", Ben means the line returning water to the reservoir. A common problem people encounter is that the returning water will cause some level of turbulence in the reservoir. Often times this results in air being sucked into the line feeding the pump. An internal baffle that breaks up the incoming flow often proves helpful.

Here is a quick pic i drew up of what i'm working with. I have a lucite(plexiglass) box i i'm going to construct as soon as i cut the hole for my fans in it. I have a Chosen Atman 306 500gph pump that is submersible but doesn't have an intake valve so i plan on epoxying its intake to the bottom of the res. and drilling a hole for the water be drawn through so it is a tight fit with the pump intake and lucite bottom of the res. Then i have my pump going directly into my heatercore which has to extended copper pipes out of it. I have two Vantec Stealth 120mm fans i will be putting on one side of the radiator with a shroud to pull air through or push through haven't decidied yet. Then just two tubes leaving my box going to my computer which will be sitting right next to the box or even ontop of it. Critique please :)
http://www.imgmag.net/images/shreder/Watercooling.jpg

That all sounds good.

dimensions on your drawing would have been helpful. I like the pump mount and the copper piping to the rad. the fans should pull, not push.

Res size doesn't matter, however with a large enough res the water weight can do a good job of priming your entire system if you arrange it right. It is nice to pour water into your system, plug in the pump, and walk away without lighting things on FIRE (cha cha cha).