The rule of thumb I usually use is that one pixel on the texture map must turn out to be smaller than one pixel in the render.
Example: I render at 2400x1800, and there's a human figure standing upright, visible from head to toe.
His body won't be more than about 1200 pixels high. A body texture map that's 2048x2048 pixels will definitely be large enough. 1024x1024 would be somewhat small (chances are that one pixel of the map gets smeared out over multiple pixels in the final render), while 4096x4096 is overkill.

I've recorded a simple Photoshop action that I can apply to folders (including subfolders). Most of the textures I bought are at 4096 square, and that Photoshop batch creates 2048x2048 and 1024x1024 downsized copies.
Which of the texture sizes I use depends of course on the scene. A background figure needs no more than 1024x1024, while foreground figures need 2048x2048 or even 4096x4096 (portraits!).

I admit, it's playing by ear, but it works out pretty well.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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