Approx 4" x 4" the ones we used . :)
You only need to cover the rebar if it is shorter than the tallest person on the job site or if there is a possibility that someone can trip and impale themselves on the exposed rebar.
Please check you local building codes for the specific application involved.
The short answer is NO. I normally drive the bars without a cap to the desired depth THEN gently tap the cap on. If the soil is soft or sandy you might get by with driving the bar with cap installed but if the soil is hard you will drive the cap right through the rebar.
Thank you for your recent inquiry with The Home Depot, john. This rebar cap is only carried in select Home Depot stores and not online. Pricing will vary. If they are not carried in your store or in the quantity you need, a specialist in your store may be able to special order them for you. Special orders are not available online. We appreciate your business and look forward to serving you in the future. Thank you for shopping Home Depot