I have an Asus M2N-E motherboard, currently with (qty 1) DDR2 module Kingston KVR533D2E4/1G. The board has 4 total slots.
Going to upgrade and I'm wondering if I can mix sizes -- can I add a 2G module and keep the existing 1G?
Yes. You can have different sizes, and even in most cases, different speeds (Not suggested though). The only thing to really look out for is that they are compatible. If you're putting in the same brand/model, just different size, I wouldn't see a problem.
EDIT: This may also be helpful: Can I mix and match RAM?
You can. However, do notice that on dual-channel memory board, this will decrease memory access speed.