So, do you wanna make spatial smoothing with FSL?
You can use:
$ fslmaths original.nii -kernel gauss 2.1233226 -fmean smoothed.nii
The gaussian kernel takes its argument as sigma in mm instead of the FWHM, but you can see how to convert between these two values here: http://mathworld.wolfram.com/GaussianFunction.html
Basically, divide your FWHM (in mm) by 2.3548 to get sigma.