Anything decent will do, 2-3 cores, 1GB RAM, 20GB disk space, and 1TB of bandwidth. Of course, these figures are rough estimates. A server owner around may be able to give you a better estimate.
1GB, 1-2 cores. Pocketmine is single cored last time I remembered, but you still need a core in theory for the OS to prevent cycle stealing, and 10gb is plenty. 20gb is overkill. 10GB SDD Preferred.
I have 10 mini-game servers with only 512mb ram each and they run pretty well with 30-40 on (But that's with Steadfast not PM 1.4) I will definitely have to upgrade their ram when I update them to the new API.
Some of my servers are hosted by PocketServer, some by DigitalOcean, and a lot of them in my own little data center in the garage.
what os and settings do you have for your servers running? i'm on win7 and keep getting clients hung up after about 4 players and 1wk of play later.