Although I wouldn't argue that 5/8" was the most widely sold size - there must be a reason why some of the Sheffield Manufacturers seemed to specialise in the larger blades. I don't recall seeing many 5/8" W&B for example whereas you see lots in the 7/8" and bigger varieties.

I started off with a 5/8" and the majority of my collection are in the 6/8" - slightly smaller or slightly bigger range that I find comfortable. The deciding factor for me is the size/weight factor - I don't find small/light blades comfortable to use, there needs to be a reasonable spine width preferably.