How big is the rift?

I'm actually positive there's got to be a way to calculate this since one Teemo (100 units) is already a measurement. Figure out Teemo's diameter, calculate the # of units from end to end, there's your answer.

I mean if its anything to go off of, the area covered in the fighting in "A new Dawn" is pretty darn big, added to the fact the nexus was still far away.

It doesn't exist.

I mean, as far as I know, Summoner's Rift isn't actually a place. None of the maps are.

Or maybe they are?

Riot really hasn't been very clear on the matter.

I think either the rift is an ancient battleground somewhere near the middle of the continent(probably close to demacia, considering the foliage), or if we consider old lore, maybe even a created pocket demension.

1. Find the size of Teemo
2. Find how many Teemos long the map is
3. Estimate dimensions

Given the fact it's two ruined military bases bases on a mountain top, it stands to reason the area encompassing the bases, river and forest is at least town sized or bigger. Baron's pit is probably the size of a large house on its own, while its territory in to the river would encompass a street of equivalent size to the house or larger.

I guess.

Summoner's Rift? Isn't that in the theatrical video for 2015 highlighting Ahri?

I'd be willing to bet about 2 football fields - half of one covering the top lane third, an entire covering the mid third, and the other half covering the bot lane third. No, not European Football, but AMERICAN. It would be 57,600 feet².

So, my estimate would be 115,200 feet² or something along those lines.