Good question, lots of different elements to answer.
1. Use 'pixels' as your ruler unit in Photoshop. Games work in Pixels for texture space, real world units would give you an insanely large texture!
2. Use power of two textures- eg 128x128, 256x256, 512x512, 1024x1024 etc. This is recommended due to the way computers process textures behind the scenes. Not doing this will result in sub-optimal performance.
3. Establish basic rules for how dense you want your texels to appear on each object. More important than mega-high resolution on everything is consistency within the world. If an asset never takes up more than 128x128 pixels on screen it doesn't need to have more than 128x128 pixels in its sprite, right? The same goes for larger objects and larger sizes. Visual fidelity in individual assets is not as important as visual fidelity across the entire scene. Calculating this can be tricky unless you know the resolution of the device you are publishing on.
4. Remember that bigger textures take up more memory! This not only increases download size, but also the memory footprint when running a game. If you are running on an older OS device, loading all these large textures at once can cause a memory spike that crashes your app. This is something you really want to plan for and avoid.
5. The texture atlas suggestion is good- this will conserve texture usage, reduce load times and reduce draw calls (very important on mobile devices!) Each different material in a scene will ask the GPU to draw it, which takes time. Consolidating these into a single texture sheet/material means that rather that asking the GPU for 20 things you are asking for a single one, which really helps performance.
Hope that helps.
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