Photoshop :: Creating 3D Cube With Custom Texture In CS6

How to create a 3d box in CS6 with custom texture. Before explaining my problem, i saw couple of good 3d onject videos already and i am close to my target but yet not finished
 
Problem:
 
I would like to create a 3d product box with custom textures, reflection and shadows. I have my top, front, back and side images in high quality.
 
My Solution:
 
1- With my box TOP image, i create the 3d layer.
2- Because it creates the box with the top image looking front, i position it as the top image comes to the top of the box.
3- I resize the box to the dimensions i would like to.
4- Side, front and back textures are missing. As its a custom made box, i can only add all of them in one image so i merge them vertically in a file and add the texture in 3d section
5- With UV scaling, i fit the image perfectly
6- Adding light and reflections
7- Render
 
Now the main problem is, my images have the exact measures of the real product box and i would like to create a box with these dimensions.
 
1- How can i play with the dimensions of the 3d box with numbers, such as pixel or cm ? As i can see, its only with %. Or is there an option so when i add the second image (side, front and back, merged), box's length dimension changes accordingly?

2- How can i create 3d box and add different textures to different part of the box? I found CUBE MESH PRESET in the forums but no download link, EVEN IN THE DISCCUSION itself

3- I have a i5-3570 , 8GB Ram, 1GB Radeon HD7750 at work, newly built. I work with high resolution files, and when i create my box and render it, some parts come out blur, why this is happening? At home i have a older PC, and it doesnt happen that much. Where i can modify the render quality or such a thing if it exists?