Samsungs often have a behavior where one setting somewhere causes other settings to be greyed out. In other cases it depends on the input signal. Additionally, they frequently come with the "sharpness" setting somewhere funny, which messes up a computer's display image. Here are details on each:
This is one example - with my MacBook connected to HDMI 3, the Picture Size "Screen Fit" option is not available, and this is the one you need - where it the input displays dot-for-dot on the panel. It turned out that my Mac was sending a 1080p signal (as it should be), but the Samsung would only show the other options if the input signal was 1080i (or 720p). So if you temporarily switch to one of those output modes, you will be able to select the settings you want. Then you can switch back to the correct output mode, and settings you are not allowed to change will still remember the values you set when you were allowed to change them.
Many Samsung TV's will grey-out most of their settings options if the picture mode is not Standard. Some will have one or two modes besides Standard that allow extra settings. Game mode and Movie mode are sometimes ones that will allow extra settings. Again as above, switch to a mode that allows you to make the settings you want, then switch back to the mode you want, and those settings should "stick" even if you can't change them any more.
This is separate from the greyed-out options issue, but related to the fact that computer output often looks terrible on an otherwise great Samsung TV. Frequently as Samsung TV will not have its "Sharpness" at the default. When sharpness is turned "up" (higher number than default, or indicator to the right of default), the TV will munge the picture to make it "better" from a standpoint of watching a TV show. But anything different than default is worse for text. Try the middle setting, and if that isn't better than anything else than try the left most or lowest setting.