The size of the sticks really doesn't matter as long as you don't surpass the maximum the board can handle. Most modern boards support at least 16 GB with 4 slots, which basically amounts to 4 GB per stick.
I should also point out that dual channel doesn't just mean two sticks. It means multiples of two sticks. You could easily buy a second set of 2x2 GB to match what you already have.
I'd also like to add that dual channel works best with identical modules of RAMs. So if you're planning to extend your current 4 Gb by another 2x2GB as MadAdmiral suggested, make sure you buy the exact same brand and model you currently have.
Also, I hope you know that for normal uses like intensive gaming, media applications etc., anything more than 4GB is unnecessary.
As an example, CoD: Black Ops (which sports some serious memory leaks), consumes about 1-1.5 GB RAM on my system (most games don't get close to that), Windows 7 would need about 700 MB - 1GB of RAM (worst case), and say you have firefox and music playing (not sure why you'd want to surf the net or play music while gaming, but just in case) and some other random applications (anti-virus etc.) which consume another 1 GB (which is on the higher side...i'm being generous), the total RAM your system is consuming is under 4Gb. And this is a worst case scenario. You won't need more than 4GB.