I've bought a few mylar bags a while ago, and 
needless to say, I stopped using them after a 
day (didn't like the glossy, oily look).  
Mylar's static cling will hold onto the cels and 
keep them from moving around in the bag, but the 
static cling might be a little too strong.  A 
few people on this board already reported losing 
paint specks via static cling to mylar bags (it 
doesn't strip away large chunks of paint or 
anything like that).  Some people also recommend 
backing cels with tissue/rice paper before 
placing them in mylar bags, but that's not good 
either since paper of any kind will stick to the 
cel over time.  (basically, if it's made from a 
tree, it will stick to paint)

Granted, I'm assuming the majority of people 
won't have these problems.  If you want to go 
with purchasing mylar bags, just measure your 
cels, add a half inch to the dimensions, and 
find the closest/next larger bag size.  If you 
can't find 10" x 13", then 11.5" x 14.5" would 
be it.