It absolutely depends what you are mining with. Slower hardware will get more stale shares.
I have a 4650 that mines at about 22MH/s, and it gets 7% stale shares. But my 5850 gets 0.36% stale shares. Regardless, if you're using deepbit, he pays you for those stales anyway.
Oh, that's interesting.
My 5% rate was with a 5770 at around 150 MH/s
Perhaps the stale rate from Deepbit is so high because blocks are solved much quicker? 5% seems very high.
No, that's global. If global hashrate goes up, blocks are found faster, thus drop shares increase regarding your hashing output. The size of a single pool doesn't influence that, since you'll drop a share once a block is found anywhere else in the network, for the pool has to build up a new block header.