Notes:
SH3D can Render upto 16384 x 16384 pixrls, so you have plenty of space to improve.

If you plan on printing, you divide the number of pixels of rendered image by DPI (dot per inch) of printer to get print size.
Example:
Rendered image 6000x3000 pixels
DPI = 300 (press quality)
Size in Inch 6000/300 x 3000/300 = 20"x10"

Getting photo realistic quality from renders requires practice not to mention a lot of test renders even with experienced people to get elements right (interior lights, sun, sky, textures, reflections, indirect shadows, shades ... etc.

Cheers