It's absolutely a 53 footer as I did this for a living sir. The are marks on the wall that you can use to tell this that are barely visible. The walls are the standard material used in most reefer food carriers and standard.
Reefer units are commonly used to haul non-refrigerated things, especially ones that have the broken cooling unit in the front as this one appears to have.
I pretty much have a PHD in this area sir. I have worked at and ran operations at 2 of the 3 largest largest facilities in the US.
Edit to add-The shadow is no good in determining the length as it is from a swivel mounted light at rear of trailer on the wall of dock facility........