Good;
A 13, 14, and 15 are the most common front sprockets on just about everything dirt, whether a bike or quad. A 12 and 16 are very rare. So stay whithin those parameters.
45 to 51 are the most common rear sprockets for dirt bikes. Motards and street can run smaller for more top end.
So, like HC#42 said, you need to change your gearing based on what you ride and where.
I would suggest 14/50 for sand.
As far as chains go, usually one tooth on the front like in 13/50 and 14/50 can be adjusted for in the same chain without too much rear wheelbase change. Same goes for 14/50 and 14/47 but moves the rear wheel much farther front to back changing the wheelbase more. Now 13/47 to 14/50 would more than likely not work with the same chain.
Why worry about wheelbase? Well, in the trails a middle to more forward placement of the rear axle in the swingarm will give you a more reactive and nimble steering action since the wheelbase in shorter. But watch out for the wheelies
For drags and racing in the sand where a paddle gives the ultimate in hook-up, moving the axle as far back on the swingarm (or using a longer swingarm) will give better nose down input since the wheelbase is longer. But the already weak steering finese of the KX500 suffers for it. Also, depending on how big the paddles are (overall diameter) and the rim (size 18,19) the best is to move the rear wheel all the way back as far as possible. Then plan on having a seperate chain/sprocket set for your other riding styles.
Tire Size? The Kings Turbo 8 scoop sand paddle that I use comes in 18" and 19" sizes. The overall diameter of the 18" is 28" (CR500) and the overall diameter of the 19" is 28 1/2" (KX500). The CR500 has a shorter swingarm with less room so the rear tire has to be set as far back on the swingarm or the paddle hit it.
Less brains than experience talkin' ...
Sly