Read 'The Bible"
Nothing is too small for grounding, until you have a problem.
Mother's R-56 antenna manual answers the simple question. There are only two hundred or so detailed technical pages on the subject.
In summary:
1. Bond -everything- together with the biggest wire possible, avoiding bends. Lightning is lazy and will only turn if it has no other choice. The ground wire is a suggestion for lightning, not a rule. Lightning already traveled thousands of feet through the air, so jumping another five feet to the air conditioner is easier than following a sharply bent wire.
2. Bond at least one 8' ground rod to the antenna system via #6 (#4 is better) and to your building electrical ground. More ground rods ten feet apart are better. Bare wire buried is also a grounding surface.
3. If possible, ground all coax at the top, every 50' coming down the tower, and always before making the turn into a building.
4. Install lightning arrestors on all coax and bond them to your grounding system.
5. If in doubt, re-read items 1 through 4.
If you think you have a failed ground, call your local rock quarry and find out who does their annual MSHA testing. That electrician has the special tools to check your grounds. Most radio guys don't.
Personally, I have three ten foot ground rods buried in my yard in a fifteen foot triangle connected by bare #2 copper and exothermic welds. One #4 connects to my radio equipment ground bar, one #4 to my 30' tower next to the house, and another #4 to my electrical entrance. Most of the copper was bought by the pound from a recycler, The insulation had been removed, and that was perfect for my needs. It wasn't bright and shiny, but after a week in the ground would't be anyway.
Overkill, but a couple years ago, my utility power drop neutral failed on the pole end. The voltage imbalance would normally be fatal to refrigerators and TV's, but because of my 'overkill' bonded grounding system the only way I knew of the failure was the sagging power line. The amperage flowing into the ground would have melted a #10 wire connecting my radio ground system to the house utility ground. That's why the NFPA code is so strict.
Another grounding story: a new government client said they were having lightning problems at their office. Before I could go out on an inspection another lightning strike traveled down the tower, jumped to the electrical system, blew up the Sheriff's microwave, and jumped between four metal filing cabinets before finding the main (copper) water line. I will summarize the secretary's description as impressive, thereafter necessitating a trip to the ladies room.
The 199' tower was ten years old, and the installer (long out of business) had grounded it with a single #6 wire on one leg. Tugging on the wire confirmed my suspicions: it was two feet long and went nowhere. Since the whole area had been recently paved in concrete, the only solution was drilling a 40' copper pipe into the ground and bonding/grounding everything to R-56 specs. A qualified electrician corrected the local building maintenance staff's 'repairs'. That was seven years ago and the tower still gets hit once a year. The Sheriff reports that his seven year old microwave is still working.