How big of a 5er can i fit in my driveway??

Ok. Stupid question. But I dont want to go out and buy a 40' rv and not be able to get it in driveway. I know there are probably a few factors to consider like obstacles such as mailboxes and trees, etc. I had a 28' th that i could get into the driveway pretty well. I sometimes had to take a couple tries. I had to maneuver the th between a mailbox and a fence at the end of my driveway.Its a single car width driveway at the end and it widens to 2 car .I think Im gonna make my mailbox removable so i can get a bigger 5er and make it easier to back in. My other rv was not a 5er either which will make it easier. I think if i do that then its just a matter of how long the driveway is. I dont think ill have an issue clearing anything. When i had my th in the driveway hooked up to the truck, i never measured to see how much length was left. But I could open the garage door on the th and still have room. Across the road from my driveway is just trees and grass. Kind of hard to explain I guess. Is there anyway to do this?