I'd suggest going with a 1 GB droplet. You might be able to get away with the 512 MB, but the larger one will ensure that you can deal with any traffic spikes. If you use the 512 MB droplet, I'd recommend add a swap file:
It will give you a bit of a buffer if MySQL starts using a lot of memory.
Swap space can be used as an "overflow" area for your system when you run out of RAM. The operating system can store data that would normally be kept in RAM on the hard drive in a specially formatted file. In this guide, we'll demonstrate how to create and use one of these files in Ubuntu 14.04.