I know that we get stock lengths of HSS that are 20', 24', 40', and 48'. As noted you can get lengths longer than this but you will have to wait for a mill run of whatever shape(s) you are wanting. Depending on how common the shape is the mill runs may be anywhere from a week to 12 or more weeks apart.
If shipping by road anything over 53' will require a permit (assuming you are in the US). 60 ft lengths are definitely done as mill runs.
Plates will depend on how thick it is, 1/2" or less is typically coil stock so you can order whatever length you need from the supplier, but it will need to be small enough to be shipped, 60ft is typically a max length. If it is thicker than 1/2 inch it is typically discrete plate and would be stocked in 20ft and possibly 40ft lengths. Again you could get mill runs to a specific length.