How big a hole do I have to drill in concrete to embed rebar in it?
I have been working with and around concrete for 20 years now. One thing is you cannot put cmu block or any other stucture on just a thin concrete pad. You must have a load bearing surface (footer) , and the specs will vary depending on soil type, frost line, and load. Second, if you wanted to dowel in to concrete you must use epoxy to anchor them. If using #4 rebar, i would go slightly larger on the bit (5/8") to allow enough epoxy for bonding and an easier initial fit.