In addition to the great page Keys gave, it might 
help to think this way (if you don't need precise 
measurements).

A4 is nearly the same as 8.5x11, but a little 
longer and a little narrower.  B4 is larger, I 
typically think of it as "legal" size, although 
it's acutally fairly wide.  Going from there, as 
the number goes down the size increases by 2x, so 
A3 is twice the size of A4 (like how 11x17 paper 
is twice 8.5x11), and B3 is twice the size of 
B4.  Likewise, A5 is 1/2 of A4, and B5 is 1/2 the 
size of B4.  

Most pencil boards (shitajiki) and Japanese 
notebooks are B5. Regular cels are somewhat wider 
than A4, and they call the size of the books that 
hold average size cels A4L or Magazine files.

Jennifer