I think block sizes should be increased as & when it is required. Maybe 2MB for a while & then when the network dictates it increase it again to 4MB & so on.
I think it's ridiculous to be thinking about 8MB & 20MB blocks now.
This creates a chicken and egg problem. Large projects utilizing blockchain are not made because there is no room and the limit is not raised because blocks are not filled to limit. There simply needs to be enough headroom and a clear schedule for predictability.
Jeff Garzik puts it very well here:
https://www.youtube.com/watch?v=TgjrS-BPWDQ&feature=youtu.be&t=12667My personal opinion is that a blocksize limit is not needed. Simple soft limit set in client config file would be enough. (like 250kB/500kB/750kB soft limits before). You can see from blocksize history that most people just use the default value and the transaction rate increases whenever a new client that ups the default value is released.
Before this schism most of people on BCT didn't even have a clue that a blocksize limit existed. But given how strong feelings it causes now I'd be happy to go with initial 8 MB limit with a predictable schedule. (BIP 101) I just think anything too close to actual blocksize will stifle and slow down the growth of Bitcoin.