What size gravel in concrete?

[Al Givitago]

I've dug a trench 15 ft long by 18" deep by 18" wide along the bottom of my garden. I want to fill it with concrete to stop my garden falling on my neighbours drive. (The bottom of the trench is now 3" lower than my neighbours garden) 'm going to bang rebar down about a foot from the bottom of the trench. Could someone tell me if I need 10mm or 20mm gravel in the ballast.
Any advice appreciated

 

[foxhole]

Aggregate for conrete is generally between 10-40mm.