Let's say all... 5(?) programmers of a development team are all relatively new (maybe experience working on 2 semi-complex games). What is a reasonable game size and complexity level to expect them all to be able to make together? We're also assuming that nobody on the team is a genius or programming prodigy, just average people with a good amount of knowledge and decent experience.

Also, I've read several articles about why you should not consider making a Massively Multiplayer Online game, but I'd like to know, based on your opinions, what is the point a game goes from Multiplayer to Massively Multiplayer.

Perhaps counter-intuitively, adding more programmers to a project actually decreases the amount of complexity the team can handle in the short term.

This is because people need to learn how to work both separately (on different things) and together (on things that cooperate and integrate with each other). This is a long-standing truth of software development, but almost everyone ignores it.

Give that team a few years together and they can do wonderful things. But until they have all "clicked" with each other, expect the output of a 5-person team to be strictly less than (5 x one person's output).

Is there a point at which this begins having the reverse effect? For example, some articles that I read often cite the amount of staff members needed to work on a large scale project, implying that having, say, 50 programmers, is better than working alone or with a smaller group. Of course I'm inclined to believe this, being that I'm talking specifically about large scale games like GTA or WoW for this example.

Edit: Then again, this could just be the effect of everyone involved being professionals with extensive knowledge and expertise, but I'm sure that has nothing to do with it.

What is a reasonable game size and complexity level to expect them all to be able to make together?

Not an MMO.

EDIT: This, https://www.gamedev.net/topic/687345-i-have-ideas-you-have-skills/#entry5336492

Idea guy? You expect experienced people to throw themselves at you so you can practice your directing skills because you can't contribute anything else?

Learn how to do one of those things you're searching for first.

What is a reasonable game size and complexity level to expect them all to be able to make together?

Not an MMO.

EDIT: This, https://www.gamedev.net/topic/687345-i-have-ideas-you-have-skills/#entry5336492

Idea guy? You expect experienced people to throw themselves at you so you can practice your directing skills because you can't contribute anything else?

Learn how to do one of those things you're searching for first.

I'd like to know, based on your opinions, what is the point a game goes from Multiplayer to Massively Multiplayer.

The "Massively" was added when games started crossing the 100,000 concurrent player boundary.
Knowledgeable programmers using off-the-shelf tools can support double-digit number of players easily. Multi-user games have been around since the 1970s. The Networking forum FAQ includes an example of such a server written in an afternoon.
Supporting a few thousand concurrent players is more difficult, but still generally not overly complex. With a little bit of work the biggest problems can be dealt with. The n-squared issue is the biggest killer -- players and clients within range of each other all need to notify each other and communication grows exponentially -- but at these levels it can usually be dealt with by some clever spatial partitioning.
Transitioning from thousands to tens of thousands can often be done by just adding another server instance. Amazon, Google, and other infrastructure providers can do this automatically. Spin up another service and all is well.
But when you start to cross over the bigger boundary, somewhere approaching 100,000 concurrent users, there is an incredible transformation that takes place. It is no longer a case of just "add another instance". The complexity of managing the infrastructure is a major task in itself. That's the transition that the business side considers "massively multiplayer".

Sadly for players, "MMO" and "persistent online world" are often synonymous. The term "MMO" is also used by players whenever a few hundred people are in the world. A slither.io server with 500 players is called "MMO" even though it is nothing of the sort.