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.
If the only difference is in size then likely Yes! Check Asus M2N-E motherboard - Memory/Device Support page for compatible memory modules.
Typically your motherboard manual will indicate compatibility with different memory modules and some order in which to populate the DIMM slots. Motherboards which support Dual-channel architecture can run two or more DDR type memory modules in dual-channel configuration. Theoretically, dual-channel configurations double the memory bandwidth when compared to single-channel configurations though it would seem that this may result in little to no real world performance increases (see this article)
DIMM Socket Color Codes:
Motherboards supporting dual-channel memory layouts typically have color-coded DIMM sockets. Coloring schemes are not standardized and have opposing meanings, depending on the motherboard manufacturer's intentions and actual motherboard design.
Matching colored DIMM sockets may indicate one of two possibilities depending on the motherboard:
Guidelines for dissimilar memory modules: