When to harvest apples?

The house we're renting has an apple tree in the backyard (whooo!). It's got tons of small apples (a little over an inch in diameter) already, and a lot of them are already turning reddish.

Should I harvest based on size or color? Not sure if this is the biggest they're going to get, or if I should wait some more.

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Oh god I don't know how to rotate it. But you get the idea?