With a 6" Dovetail a .019 is gonna be a lot easier to see. I shot a .010 MBG Ascent on a 4" dovetail and swapped out to .019 pin. I had no problems shooting out to 100 yds with it with the bigger pin.
Im not certain I've explained this well but bow sight isn't a rifle scope and what I mean by that is with a rifle scope that's adjusted properly you can clearly see the both the target and the crosshairs and the size of the crosshairs can have a direct impact on group size. With a bowsite it's impossible to keep both the pin and target in focus at the same time. This isn't debatable as the human eye can't possibly focus on one item 3' away and another at 60' to 300'. All that said if you're focused on the spot you want to hit the pin should just be a reference point in between your eye and the target.. With that said pin size should be something you can see in your peripheral vision and within reason size, in this case, doesn't matter! I personally have shot .019 pins from 20 - 50 and a .010 at 60+ on my slider since 2009 but my next sight will have all .019 pins. I do have trouble on occasion seeing that small pin but I've never had an issue with any pin "covering up" the spot at any distance when I'm properly focused on the spot rather than the pin.
I didn't read all the replies, but you can go custom and get larger pins for your closer yardages which will more than likely be the pins you use at low light so they would be better to have big, but can get .010 for the extended distances.