I think the best thing to deal with an open-ended market sizing case like this is to first bound the problem. Identify the major industry driver for the use of freight trucks and clarify with your interview: "I know there are X, Y, Z use of freight trucks, based on A, B, C, is it okay if I estimate the #freight required for the transportation of consumer goods?" If your interview rejects every attempt of yours to bound the problem, then you may want to approach it differently.
All Marketing questions are best solved by first understanding whether it is a demand-driven question or a supply-constrained case.
Are the #Freight trucks in the US so readily available that the demand for it can be matched by the supply of freight trucks? You could argue yes/no. I would say no because if supply is made to always match demand, then you might have to account for all major demands for freight trucks. Besides, arguing for yes is just going to complicate the estimation because now you have to account for all major demands for freight trucks. So this may not be best solved through the demand-driven approach.
It is supply-constrained if the demand for it does not automatically drive up its supply. For instance, the demand for American Visas in my country does not drive up the supply of AmE visa at the embassy (else we would have all left our country lmao).
You can argue that this case is supply-constrained. Else how would you make a case for supply chain management?
Now settling for the supply-constrained approach, you can now say #Freight trucks in US/year is the function of production capacity of factories in US/year + #Freight trucks imported/year + the #Freight trucks available from the previous year. (Assuming that damaged trucks are not part of the equation). Something like that. That should be manageable and should help you get to a realistic number.
Another approach is to use relative thinking. Compare the #cars on a typical highway to the plausible #frieght trucks on a typical highway/week multiplied by a correction factor (to account for freight trucks making multiple trips a week).
Make sure to cross-check your answer.
Estimation cases were always a pain in the ___ for me until I found FirmsConsulting. If you want to more details on the demand/supply approach I summarized above, you can find it here: