With PPLNS (and most other reward methods), the expected value per share is always:
Code:
block reward * (1 - pool fee) / difficulty
PPLNS works roughly like this:
Whenever a block is found, your proportion of shares in the last N shares (usually in the last M shifts) is calculated. If you submitted 1% of the shares, you'll get 1% of the block reward (minus pool fees). The shares are not reset when a block is found. If the pool finds a new block after a couple of seconds, you'll get once again 1% of the block reward.
Say mining pool B has twice the hash rate of mining pool A. At pool B, you'll get only half of what you'd get in pool A when a block is found. However, pool B will find twice as many blocks as pool A. If both pools have the same fees, you'll earn the same at both given enough time.
The only aspect where "size matters" is variance. A big pool will find blocks faster. So even if a block took 6 * difficulty shares to find, it will last only a couple of hours. Small pools can go days or even weeks without finding a block. The expected earnings over time will still be the same, but the earnings in a big pool will be more steady.