I have a 86 W250 that needs new hanger brackets for the leaf springs what bolt size would I need to attach the bracket to the frame.

Measure the holes, see what size they are, get bolts the fit snug. Grade 8 wouldn't be a bad plan there either.

I use 1/2". Easier to find vs 7/16"... and stronger.
Swish the holes out a little bit.