76 VanDura - Starter solenoid positive nut size
 

Vehicle: 1976 GMC Vandura 3/4 ton. 4bbl carb.

My van was having a rough time starting, starter sounded like it was going to engage but would not crank the engine.
I crawled under the van and found there is no nut on the positive starter solenoid lead from the battery, and a very beautiful yet unsettling shower of sparks every time I tried cranking it.
Just wondering if anyone here knows what size nut goes on the battery positive stud on the starter solenoid.
It's getting annoying crawling under the van and jumping the starter since my van has low ground clearance and i can barely get a screwdriver in there to jump it without a jack.

Also, since this nut fell off and I know I had a locking washer on it, any tips to keep this from falling off again?