It has a lot to do with personal preference and what you want the blade to do. A lot of people have rotations of razors of different sizes, thinner razors for more intricate work (like under the nose for example) and larger ones for more demanding work (coarse bristles, lots of hair, etc).
Its hard to consider the width of the blade in isolation without taking the grind into account. For wiry thick beards lots of people like a thicker blade - coupled with a stiffer grind. However, a slight hollow grind on a 5/8 will make it much heavier than a full hollow ground 5/8, and tang lengths in 5/8 and 6/9 razors do not vary that much (I measure a lot of them - hardly ever anything in it up to around 1/4 of an inch).
In general it is best to start with something not too small - its easier to judge angles if the blade is a bit thicker. Also - and pretty obviously - thicker blades will hold more lather before needing to be rinsed.
FWIW I think a 6/8 is a pretty good blade width - a little trickier under the nose, maybe, but whats an eighth of an inch between friends?! My (current) favourite is a 6/8 revisor, but I use a 4/8 Ern and a 5/8 Dovo too. Horses for courses!
Regards,
Neil