HTML source code to set for laptop and mobile

Hello e-learning heroes,
Hope all of you are doing well. I would need some help from you guys.

Is it possible in HTML source code to define separate sizes for Laptop and Mobile? The current setup we are using isn’t responsive - whatever we add applies uniformly. For example, if we set the dimensions to 1000px width and 350px height, it may look fine on mobile but appear too small on a laptop. And if we adjust the size for the laptop, it could end up being too large or awkward on mobile. I’m not sure where the issue lies, and I’d really appreciate your help with this. 

Also, to add I have used Rise web output, and this is for a webpage. Let me know if you need more information. Thank you all so much.