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.
The keyboard methods of resizing windows are nice, but only a few users know about them. Everyone knows about resize borders, and they should be made usable.
*** This bug has been marked as a duplicate of bug 11808 ***