Disclaimer: I am a total Redis newb. I'm sure this could be done better.
TL;DR The 4GB droplet is oddly almost 2x faster than smaller and bigger droplets (which are otherwise pretty consistent).
Some quick benchmarks for running Redis on DigitalOcean. All tests were run on the same "Ubuntu 14.04 x64" provisioned instance resized between the different droplet sizes. The benchmark was run twice (once in the afternoon PST, and once in the morning PST). Benchmark executed as:
> redis-benchmark -q -n 100000