Just load an image file with that resolution. Take out MSPaint, set the image size to 1024x1024, bucket fill it with one color and save it. Then import this image into MED, delete any created mipmaps, and save the model. If this works, use the next larger image size, 2048x2048 and repeat. Note that they are always a power of 2. Textures should always be a power of 2 for the width and height (but it doesn't have to be a square).