We've recently posted Don Srull's article on "Estimating Rubber Cross-Section Requirements"  look at reply #6 Click Here
Sky9pilot

Rubber size has alot to do with personal taste.
Thinner rubber will turn longer, but with less power. Good for indoor stuff.  More time, less rafters rash. 

Wider stuff has more "Power Burst" but less turns. Tends to make her climb higher, but runs out of steam faster. Works great if your plane is a tad heavy.

For most peanut scale, 3/32 is great.  I use 1/8 [4/32] for most small stuff. You can always use more than one loop. You probly already know this, but, 2 strands is 1 loop.

Each plane reacts to different rubber sizes and prop sizes differently.   If you can, try to get a couple different sizes and try them all.  Personally, I go for the thinnest rubber I can get away with and LONG loops of it.  Alot of folks go just the opposite.

This might help put it into perspective. Please remember, this is just a guide, not cast in stone. Each motor is a bit different. This chart will get you near max turns. [Close to breaking] Grant yourself some margin for error so as not to blow out the tail of your plane. Personally, I like Super Sport rubber, if you can find it. Call or email before you put out your cash. Dont forget to lube that motor.  A jar of good lube will last forever.

Marc,

Checkout the Supplies section....there are several FreeFlight suppliers listed there: Shorty's Basement, A2Z, etc. that offer rubber in many differing sizes.

Here's the link: http://www.stickandtissue.com/cgi-bin/yabb2/YaBB.pl?board=supplies

Tom