Are you swaying more towards one than the other? The small things that can make a difference are the style, I believe the sellier style appears larger than the retourne style, also the colour, lighter ones such as gold 'advance' and appear larger than black and dark colours which retreat and appear smaller. So for example if you like the idea of a 28cm Kelly but think perhaps you ought to have a 32cm because of your height, then a 28 sellier in gold would look pretty much the same size as a 32cm retourne in black, know what I mean?
Here's a pic to show you two Bolides, the smaller orange 1923 is 29 or 30 cm against the larger 31cm dark blue Bolide yet the orange appears larger in my mind, it's not a perfect example as the handles have different drops and there's only a cm or two between them but colour does have a big impact I think: