http://www.schmetzneedles.com/learni...eedles.htm#pdf
http://www.schmetzneedles.com/learni...edle-chart.pdf
http://www.needlepointers.com/ShowAr...spx?NavID=1802
Above are a few good sites with information. I can't give you tooo much info except that the larger the number the thicker the fabrics the needle can easily sew through. Size 11 or so would be for 2 layers of cotton - size 14 would be for jeans or thick layers of fabrics. (I recently had to UP the size of my needle when sewing through several layers of polyester that had a lining in it. I took my time, lengthened the stitch to 3.0 and it worked perfectly :) )
I tried using the Microtex needle for fine metallic fabrics and it worked fine throughout my projects.
I also think it's trial and error till we get experienced enough to know better. Print off some info and take notes...before you know it - you'll know it!