Don't choose random relays. That's not good. There are lots of relays tracking and selling people's data for profit. That's a terrible strategy.

Small relays work on amethyst and other outbox model clients, but many clients haven't updated yet, so the large relay helps with reach. 

Most importantly, you want to use the relays your friends and follows use. That's doesn't need to be a large relay. It just do happens that today, users haven chosen to centralize in large relays.