This topic has been covered many times. So here we go again. If you want, for example, an 8"X10" print at 300dpi, do the math. 8" X 300 = 2400. 10" X 300 = 3000. Therefore, you would want to render your image in Lightwave at 2400 X 3000. After loading the image in Photoshop, or whatever program you use to print, setting the dpi to 300 will automatically format the image at 8"X10".
You are falling under the common misconception that dpi measures the resolution of an image. That's not correct. It is a setting of how compact you want the pixels. I know that may seem like resolution but image resolution is really the number of pixels you have to work with.