Assistance figuring out how to prevent website from resizing constantly

Hey everyone!

I'm needing some assistance trying to figure out why our website is always resizing for different monitors/computers. When I originally setup the website I created it on my Computer with a 4k monitor but now anytime a coworker goes to the site to check it out things get resized and things begin to overlap or cut pieces off in weird ways.

One of our employees's mentioned that with their monitor the website doesn't look normal until they zoom out to about 65%

Is there anything specific that is causing this and is there anything I can do to help prevent it so the website layout stays more uniform on all monitors?

Something is messed up with the style sheets right now. Everything is rendering pretty badly. 

For me, scrolling in the mobile view is causing text and image containers to jump up in size for no reason.

If i wanted to go through and fix the site would I need to go through and reset all the style settings? Or is there an easier way to go about fixing the issue?