I’m so sorry for the slow reply, I wasn’t getting most forum notifications since switching laptop, so am catching up on everything now.
Well if you want the text blocks to be 800px wide on large screens, and the full width of the page on smaller screen sizes (e.g those below 800px) you could simply set the max-width CSS property to 800px, and ensure the width property is set to auto (rather than a fixed value). That way, the blocks will be fluid on smaller screens, but no more than 800px on larger screens.
I hope that helps, and apologies again for the delay. If I’ve misunderstood your scenario, please feel free to clarify further and if possible send me a link to the page you’re working on.