How to make the lightbox appear smaller for the customer at the front end

Hello! I am doing sublimation and i print only in A4 & A3 paper. 

I have created my products in 300dpi A4 & A3 sizes which are A4 ( 3508X2480px ) & A3 ( 4961 x 3508px ).

The problem is that on the front end the stage appears very big and fills the desktop screen.

Is there any way to make the lightbox smaller with the same resolutions?

Also is this a correct approach in setting the stage at these sizes?

Thank you in advance,

Stathis