Quote:
Cover is 3 1/2" by 2 1/2" - I wonder if this could be legally mailed today that small?
Absolutely not!
There are strict size limits today. Minimum envelope sizes must be a lot larger than they used to be because the pieces have to fit into modern sorting machines that will not accept pieces that small.
Believe it or not, the US Post Office was fighting this problem back as early as 1921, with this news release:

As time went on and mail sorting equipment became more and more mechanized and computerized, the outright refusal of accepting mail in envelopes that small became necessary.