I'm thinking about upgrading my account, to Gold membership, but I can't find any information regarding the quote below in the upgrade page. Will I be able to create and manage tournamens as a Gold Member, and if so, how big tournaments?

"You must become a premium member to host tournaments on Chess.com. If you would like run a tournament, upgrade your account now!."

I'll take your question!  As a Gold Member, you can create or play in up to ten tournaments at a time.  Tournaments you create can have a maximum of twelve contestants.  There cannot be multiple rounds for the tournaments you create as a Gold member.

You can see the breakdown here.

As an aside (and I don't work for the site, just a member): I don't run into too many gold members.  I think this is primarily because most people feel you get quite a bit more in the Platinum membership for that 1.50USD difference.