if you do a normal area peak pick (Ctrl + Shift + left-drag (or Cmd + Shift + left-drag on a Mac)), then the program will automatically try and fit the peak (similarly if you picked your peaks with the Pick & Assign module). In the peakList table you should then see an LW (line width) parameter for each dimension. This is the width at the half height in Hz.
You can check how good the fit is in the SpectrumDisplay by opening the Settings and selecting to show the Peaks as “Filled Line Widths” rather than a “Cross”. Also remember that you can switch the axes, e.g with shortcut YZ, so you can open a new SpectrumDisplay in which the C rather than the N is in the z-dimension. That way you can also check the fit in the third dimension, too.
You can refit fit selected peaks with the RP or you can fit several partially overlapped peaks together with RG (these commands are also in the right hand mouse menu for selected peaks).