What should the active validator set size be?

[KaiserKarel]

Introduction

The active validator set size determines how many validators are producing and finalizing blocks. We need to determine the size for the mainnet and model testnet after that.

Tradeoffs

More validators mean fewer staked tokens per validator. The p2p performance also decreases with increased validators (the voting process for new blocks requires 2/3 validators by power to sign the block). Finally, we increase proving times by increasing the number of validators.

Proposal

From our benchmarks, we know we can scale up to 128. I do not believe that 128 is significantly more secure than 64, and not worth the overall tradeoffs. Below 64 seems like a low number with regards to decentralization, hence I propose 64 as the number of validators to use.