It depends on tne weight of the envelope not the size.
100 lb
It depends on how heavy the contents are and how big the envelope is.
7007 lb
4536kg
250 lb.
4to5 lb
It can weigh up to 45 lb.
400 lb (184kg)
Heavy as hell but really there 2000lb in a ton 7x 2000= 14,000 lb
295 kg (650 lb).
Seven tons (14,000 lb.)