Size is fine, that's how people like me have 12 GB. The only bad thing about size differences is that it can only run in single channel. Speed can also be mixed, but it will automatically clock to the lowest setting I believe.
For reference all of this is from my own experience with my PC. I'm running 1x8 and 1x4. The 8GB stick is supposed to be 1600MHz, and the 4GB stick is 1333MHz. Everything seems to be fine for me, although because its a laptop you're working with, your mileage may vary