Obviously if you have the money and the room you should build a garage big enough to store a 747 just in case you ever buy one. If money is an object the consider things like do you really need a 22 foot wide garage? Do you really need to open your slides while it's in storage? Your profile says you live in Florida. How hard is it to pull the RV out to work on it and open slides? Is it so hard that it will make the thousands upon thousands of dollars in extra expense worth it?
I currently keep my RV in an indoor storage unit. I think my unit is 14 foot wide by 42 feet long. I back the RV in and put it right up against one wall because I have a bunch of personal items in boxes store against the other wall and I store stuff behind the RV too. So I do have room on side to walk up and down between the boxes and RV and I can't walk around the other side. So if I need to open the slides I just start the RV and pull it out. Sure it would be nice to have all that extra space, but for me it wouldn't really be worth the money unless I needed all that extra space for other storage. If I were building a building that was only intended for RV storage, I'd personally make it big enough to fit the RV with enough room on the sides so I can get to things like the battery compartment and the tires in case they go flat in storage, but other than that I don't think I'd waste the money just to have space to work on it and open slides. I'll just pull it out in the driveway to do that.