You would just create a wrapper. On desktop set a pixel width at the desired size. Set the wrapper with left and right margins set to auto if you want it to remain centered on the screen (otherwise it will hang to the left). The fixed pixel size will keep anything from scaling up or down. You will get horizontal scroll bars for the content if your browser window is less that your fixed width.
On Mobile tablet set the wrapper to 100% and reset the margins to zero and everything will remain responsive for mobile breakpoints. It will cascade down so that phone is also responsive.
You will will maybe / likely get penalized by Google for an unresponsive page on desktop? This is a very odd and unadvisable request. I would try to explain the benefits of a properly designed responsive page and ask the client to articulate their goal for making it constrained.
Good luck on both fronts - it isn’t always easy and I empathize completely.
Hope it helps - Rob