ADCO recommends including the ladder but I'd always just measure the box itself at it's longest dimension and go with that. Covers, unless custom made for a specific trailer, are sized in ranges - i.e. 20' 1" to 22', 22' 1" to 24', etc, so chances are your's won't be at the extremes anyway. Obviously the covers are fabric and are quite generous so I personally wouldn't worry about the ladder, the spare tire & bracket I wouldn't cover anyway as I always dress the cover tight to the trailer's rear wall.
2012 Silverado 1500 Crew Cab2014 Coachmen Freedom Express 192RBS2003 Fleetwood Yuma * 2008 K-Z Spree 240BH-LX
2007 TrailCruiser C21RBH * 2000 Fleetwood Santa Fe
1998 Jayco 10UD * 1969 Coleman CT380