Growth Dependent Block Size Limit Could the 1MB block size limit be replaced with a growth-dependent limit? From what I have read (here, for example) the 1MB limit is to prevent spamming of the blockchain. And if/when Bitcoin becomes more widely adopted we will have to increase the limit so that the network can handle more transactions per second. 

Instead, could we have a rule like "blocks can be no larger than 1.5 times the average of the last 100 blocks"? The hope is that we could automate this spam prevention measure while still allowing for growth. Has this idea been considered anywhere else at all?

Maybe this be too large of a protocol change. Would we be able to get everyone to update their nodes to follow the slightly altered protocol? 

Thanks!
Technically, this is possible. However, it would be a hardfork, so all users would have to upgrade. In such a large ecosystem that exists around Bitcoin today, this would be rather disrupting.

Such change would also have unclear security implications. Miners would be naturally eager to accept as many transactions as possible, so a spammer may gradually push block size limit up to increase the time it takes the network to process new blocks. This would lead to an increase in orphan rate, which would make it easier to mount a 51% attack, especially with the help of selfish mining.

This idea was discussed many times on Bitcointalk. While most of the developers agree that a change would be necessary eventually, any specific plans have yet to be announced.

By the way, some altcoins, such as Monero and Cryptonite, have variable block size limit using rules similar to the ones you propose.