Many years ago I was driving along a country road and espied in a lay-by, a van, and a falconer with a BOP on his wrist! Out of curiousity, I pulled over just ahead of him, got out the car, walked slowly towards him and said "that's a fine female Sparrowhawk", to which he replied "she is a he, and he...is a Goshawk!
This was the closest that I'd ever been to an Accipter, thus I enquired to it's size and was told 17". Thus I've always been mindful of (in this extreme case) the almost "fine line" that there (can be) regarding any potential overlap...and that was standing just a few feet away, that would approximate to a circa 14% difference between the contenders.
Add on a moving bird, appearing and disappearing c100m away in less than favourable light, and you'll have a very problematic subject to deal with, equally I've found size variability between Sprawk gender, occasionally....a big male or a small female I've asked myself, again, often in less than favourable light and compounded with brevity of view.....a-a-a-a-h! the joys of Birding.