you just have to keep one pixel of your texture image from getting any bigger than one pixel of your rendered final image. So you have to know how big the object is going to be in the final render, if it stays far away and is never bigger than 100 pixels across in the render (for example) then the texture doesn’t need to be any bigger than 100 pixels (I’m assuming a one sided example for simplicity here). If you zoom right in on your object the texture is going to have to be bigger… how big depends on final image size and the size of the ‘target object’ in that render but I don’t know of any way to figure that out other than trying it out.