Likely 9/16 x 3 5/8". We do not what weight capacity the springs are or much anything about your trailer.
If you are changing the wet bolts I'd also recommend you replace the shackle bushings with good bronze units. The common OEM plastic bushings don't last very long.
Fred & Denise (RVM157) New Mexico
2007 Excel Classic 30RSO & Coach House 272XL
2007 RAM 3500, Diesel, 6Spd Auto, SWD, 4x4, CC & LB