The challenge is now to find a number for this cap. [...]
1) 20 MB is too big right now.
2) 1 TB is definitely too big. Just imagine the IBD after 2 years.
3) You're thinking too big. Think smaller. We need some room to handle the current congestion, we do not need room for 160 million users yet.
160 million users and 20 MB maximum block size (1 TB/year) as a mid-term goal is based on the present consumer HD market storage prices, but also on the idea to capture a significant (at least 10%) part of the market of Western Union and similar services (WU claims to have 1 billion clients). The remittance market is, for the time being, the most interesting one for BTC if it manages to continue to offer fees of less than ~1 USD per (simple) transaction.
The "upper cap" of 20 MB could be the mid-term cap, to be reached ~ 10 years from now. We could set a lower cap for the first 2-3 years (5 MB should be enough, or 2 MB + Segwit) because of current bandwith limitations. Or a moving cap based on speed tests like the one Franky proposes (good idea, I think).