100A is the trip rating of the breaker, not the actual connected load. When someone is asking for a '100A panel', they rarely will ever actually use the full 100A of capacity. This is why you really need to know the expected loading. And unless you have an energy code to deal with, you also need to know the voltage drop that the equipment can reasonably tolerate. Some stuff is just fine with a 10% voltage drop.
The customer might want 100A of capacity for a once in a blue moon event, eg. initial cooldown of a hot refrigerated trailer, or keeping resistance heat running on the coldest design day, and then only actually use 20A of capacity on a continuous basis. Or they might only need 20A and like the idea of having a 100A panel.
IMHO a reasonable compromise is to run a wire that _both_ has acceptable voltage drop during expected regular loading (either the calculated average load, or frequently repeated peak loads such as AC starting), and a minimum ampacity of 100A, protected by 100A at the supply side. Another reasonable compromise is to have a wire that meets the voltage drop criteria, has a calculated load less than 95A, and an ampacity of 95A protected by 100A breakers.
Another thing to keep in mind: if you are running 208V 'single phase', you have to keep in mind that this is really 2 phases of a 3 phase system, and the neutral will not balance for 120V loads. You will have voltage drop on the neutral even for _balanced_ 120V loading. Most likely with a 100A panel in a trailer they are expecting significant L-L loading, but again we don't actually know the true loads.
