Creating a static width for entries

[magnavox_23]

Hi all! I was pointed this way to possibly find help with what is probably a really basic style issue. I am wanting to know if there is a way to keep the width of my entries static across all devices? (I'd been happy with anything 600px +) My page looks great on the PC, but on a tablet, the width of my entries shrinks and it throws out the placement of my artwork.

Desktop:


Tablet:


This is the theme I am using:


I have a paid account, and I've had a play around with the customize your theme option, but no luck, and I only know very basic html and css. Any help or advice would be very appreciated!