If you right click the title bar of any window, to open a menu, from which you can select resize.
Even though it has been 7 years since the initial report and the issue is still very much active, the assigned dev says it's a low priority to him. If you know how some FOSS project still work, there's very little hope for a change anytime soon.
Have you tried changing the window title / border theme in Settings / Window Manager? There are a few themes with resize corners only provided on top and bottom, and with left/right borders 1px thick; you might have switched to that and had the frustration. But there are definitely a number of themes with comfortably thick border. (My preferred one is Smoothwall.)
For what it's worth, it works very well one-handed on laptops, this is how I've used it for years and I greatly prefer it. The click target, that is the area you have to aim for with the mouse cursor, is much bigger than even the most absurdly thick window borders, so I find it much easier to use.
But I just live with it. Honestly I don't resize windows that often. I either use the window's natural size, or maximize. In the 18 years I've used Xfce it hasn't been a big issue for me.