What is the gross vehicle weight rating of the trailer?Just divide by 2 if a single axle...4 if a tandem....
You also need to know the style of springs it has, slipper, C-hook, double eye with a shackle, and the distance between the eyes or eye and hook (opening where the bolts go)
http://www.championtrailers.comhttp://www.easternmarine.comhttp://www.etrailer.comall have good information on how to measure springs and how to figure weight capacity. I'd grease em up good with marine wheel bearing grease to keep em from rusting. I've done this on my last 2 sets of springs and the don't rust even in salt water. I squirt motor oil with a squirt can between the leaves, then coat the whole spring with wheel bearing grease before I install them. Then I spray them with either Corrosion X or Boeshield T-9 like 2 x a season and they stay corrosion free. Where I am in salt water trailer springs will rust up in about 4 seasons they are shot. Mine still look like new.