I find it noticeably more difficult to resize windows on xfce4 than other platforms.  I believe the resize border on my xfce4 is three pixels wide.  I also use a Mac, where the border is considerably wider - perhaps eight pixels.  

I found a discussion thread from three years ago complaining about single-pixel borders.  Apparently the three-pixel borders were introduced about then.  However, display resolutions have continued to increase.  I suggest that no fixed width in pixels will continue to be satisfactory.  Please make the width configurable, with the default being a certain fraction (perhaps 1/250) of the display height.