Hello,
If your monitor is not showing all of the computer content, this means the settings is wrong on the monitor side, not on the computer side.
By the way, I believe your monitor is Samsung Monitor model T24C550, not Dell, right? I cannot find a Dell monitor with that model number, but Samsung has one.
I looked up the monitor manual, looks like the settings can be accessed via:
Monitor's menu button > Picture > Picture options. Then look for size, and adjust from there to find what fits your computer's resolution. I believe you want to use the screenfit option. I attached a screenshot of this part of the instruction from the user guide for the monitor.