I know it is a hard concept to understand, but the CSV with a 4.5 gallon tank will cycle the pump just exactly the same number of times per day for house use as a system with an 80 gallon tank and no CSV. Without a CSV the pump system will just cycle on and off continuously while water is being used for long periods of time like for showers, and not cycle very much for small uses of water like toilet flushes. While the CSV and small tank system will eliminate all cycles while using water for long periods of times, and cycle occasionally for the small uses. But the overall cycles per day will be the same.
Then if you have any real long term water uses like sprinklers, heat pumps, or drip systems, the CSV and small tank will only cause one cycle. Where a "so called properly sized pressure tank" system will cycle the pump on for one minute and off for one minute, which can add up to 720 cycles in a 24 hour day. 720 cycles verses 1 cycle is a big difference for long term uses of water.
With the CSV system, as long as more than 1 GPM is being used, there is ZERO cycling as the pump will run continuously. The water will go right past the tank straight to the shower or sprinkler, so the pump doesn't know if it is a 1 gallon or a million gallon pressure tank.
Actually the more people you have in the house, and the more water is being used, the smaller the tank can be when using a CSV. If you have enough people in the house that someone, somewhere is using water all day, the pump is running all day, and the size of the tank is a moot point. We find that is nursing homes and things like that people will use water heavily certain times of the day. So the pump may run continuous from 7AM to 1PM. Then it may come on a few time between 1PM and 6PM, and run continuous again from 6PM to 10PM.
The average system with the CSV and small tank will cycle the pump from 20 to 50 times per day, which is the same as the big tank method without a CSV. This is well below the pump manufacturers limits of 100-300 cycles per day. Now if you use the CSV with a large tank it will reduce the cycling to much less than the normal cycles per day for house use only. The average cycles per day will decrease from (20 to 50) down to (5 to 20) cycles per day. However, there is always a trade off. While using the CSV with a large tank will reduce the total number of cycles per day somewhat, it will not lengthen the life of the pump enough to justify the added expense of the large tank. Other disadvantages of the large tank include space and heat requirements, not to mention the taste of having your water sitting in a rubber bag for hours before being used. However, the biggest disadvantage of a large tank is low pressure. An 80 gallon tank holds about 25 gallons of water. But when using this 25 gallons the pressure will be decreasing from 60 all the way down to 40 before the pump is started. With the CSV and small tank, the 1 gallon in the 4.5 gallon size tank gets used very quickly when turning on a shower, and the strong constant 50 PSI will be maintained for the entire length of the shower.
Although holding 50 PSI constant may not seem that much different than when a pump is cycling on at 40 and off at 60, it certainly is. People tell me the pressure feels so much stronger they no longer even need soap in the shower, as the pressure just blast the dirt off of them.
Having said all of that, you can use the CSV with any size tank you want. If you already have a large tank, by all means use it. But I am sure you will like the stronger constant pressure from the CSV and small tank system. If you have to purchase a tank, just get the small tank, as you probably have many other uses for the money you can save by not purchasing one of those large expensive tanks.