How much cement and sand to buy for a garden edging project

Roughen all areas to be resurfaced, remove any loose or unstable material. Dry mix 5 parts fine sand to 1 part portland cement. Moisten entire area to be resurfaced. Add small amounts of water, a little bit at a time, and thoroughly mix until entire batch is damp. For initial dry-pack, proper mix should hold as a clump if you squeeze together in your hand, but easily come apart if you break it up. For your resurfacing purpose, it could be a bit wetter but not liquidy. Spread out and pack firmly into area in question with a wood float, use the wood float to feather the edges of the repair into existing surface. Use a spray bottle to moisten as needed while you float it out. Screed or float out as level as you need it to be but the surface does not have to be super smooth.

Apply water-proofing membrane, then apply additional treatments (float regular cement on top of membrane then install tile?). Drypack is most often used to establish initial slope for the surface, not for applying final finish treatment to.

Concrete is a mix of large aggregate, small aggregate, and cement (a 4:2:1 ratio is a good approximation - though designed mixes will be more calculated than that).

The size of the large aggregate isn't particularly important, unless you are working in very tight spaces or around reinforcement, in which case you want suitably small aggregate.

In this case there won't be a problem at all using softball-sized rocks. Just make sure that they are clean (ie don't have soil stuck to them).

(I remember seeing a section cut through a piece of concrete at the Hoover Dam Visitor Centre, and some of the large aggregate in that was rocks up to perhaps about 8 inch / 200mm or more).