You are going to get a billion different suggestions for this question! A monitor is limited to how much detail it can display in one physical inch, so there is such a thing as too much resoltuion, which will only slow down your renders. A general rule of thimb to use is that you should make a texture map as big as you will ever see it on screen. In other words, if you have a rock texture and your camera passes right over that rock and it get's shown real close, then you need a bigger texture in order to have a quality look when you are up close. But if that rock is never going to be very close to the camera, say no more than 1 inch big on the screen, then using a large texture on that rock would be a waste of processor time. Keeping proximity to the camera in mind, if your texture looks pixelated, jagged or rough then you probably need a larger texture image. If it looks nice, you have a bit enough texture. A common misconception is to use a "high resolution" texture assuming it will increase the realism or detail of a render. This is not completely true. For example, a 4000x4000 image rendered on a character is really a waste of system resources and processor power unless you are rendering a tight shot of the head, or an extreem close up of the lips (and even then it's usually overkill). Always strive for a balance between speed and quality. There is a whole lot that could be said about this topic, but experimentation will get you the best results. Best of luck.