Some good advice given here but Gary's advice is the most important, the sub-grade is the primary consideration when engineering a road surface (which is what this is in essence). If you have a sand bedding then just a couple inches of 3/4 crushed rock would suffice if well compacted. Sand is generally well drained and does not heave in the frost like a clay or loam soil will. A clay substrate will move the closer it is to the surface and needs to be removed below the frost line (depth at which the ground freezes). If you have sand, just lay a couple inches of class 3 road base, level it out and compact and yer done. If its clay or has high clay/silt percentage then you need to determine the frost line as determined by your local climate. 6 to 12 inches should do it unless you live in North Dakota or someplace subject to long cold nights. You will want to lay a couple inches of sand down to act as an insulator between the clay sub-grade and rock base. A true road base material such as the class 3 (a Cal-Trans standard YMMV) has all crushed rock with many fractured faces that is specifically graded by percentage of weight by volume or whats called a gradation. You want a variety of sizes of aggregate to get a solid section. bigger rocks, generally 1/2" sieve to 3/4" to add weight bearing and longevity and then there is varying size smaller bits to fill the voids between the larger bits. Every state highway department has standards for road base and any material supplier will have a supply of it on hand. Don't ask for "half inch rock" tell them you want road base. I've been inspecting this poo for twenty years. ETA: a vibra-plate compactor will work fine if the lifts are only a couple inches and you have plenty of moisture. Rockhound knows whats up. .