What size nuts & bolts should be used on trestles bents?
Depends. Yeah, not helpful.
I have seen 0.5" bolts on sway braces and girts, and something near 1" bolts holding a Howe Truss together...more like rods, though; they were 15' long or more. They were in pairs. Their nuts would be near 2" across.
This is the central truss at the bottom of the Kinsol Trestle on the Koksilah River in southern Vancouver Island. Note that it has several sizes of pins, bolts, and rods, and even plates with bolts or rods in an array. What you can't see are the inset plates under the stringers through which those long vertical rods rise. Against the plates are those large 2" nuts securing and tensioning the long rods.
According to the Model Railroad Bridges & Trestles Book the rods on a Howe Truss Bridge are 2” diameter or .025” HO scale. I built my bridge using the MR book and used Tichy .02” phosphor bronze wire for the rods and .04” styrene rod for the nuts. Unless you are within 16” to 18” from the bridge you can’t tell that the nuts are round. At 3’ they are barely visible.Mel My Model Railroad http://melvineperry.blogspot.com/ Bakersfield, California I'm beginning to realize that aging is not for wimps.