I've built about a half dozen wheels, including three fat tire models. I prefer to use 14G spokes with 13G elbows, but I have done 13G to save a little money. My Ecotric 20" fat folder comes in about 2.6mm diameter on its spokes, which is 12G. EVen the front wheel is 12G.
I get the length from the inner edge of the hole to the rim at 127 mm, same as your 5 inches. Supposed to add the width of the rim and how high the top of the nipple sits above the rim, and that's usually about 2 mm.
However, replacement spokes are easier to install if they are on the short side. This works for a few spokes. If all the spokes were short, be hard to build the wheel. Anyway, if you have broken a few, you should know if the replacements had to be bent to go inside the hole, or if they swing freely just above it.
I get my spokes from this guy. He's in Chicago, but he sells on ebay. Send him a query for pricing. They probably won't believe you if you say inches. Change it to mm. Order the nipples too, as I found the chinese nipples fit poorly on spokes cut in the USA.
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