In the US the smallest size that you can mail is 5" min length x 3 1/2" minimum height. That's the same for letters and postcards.
However, different countries have a different rule for how small an envelope can be if it's sent through the post.
In the UK there doesn't seem to be a minimum size restriction as long as the address can fit on and be easily read.
- When the parcel post service started in the US in 1913, people immediately began testing the boundaries of what could be sent... with some people even posting their children!
Mr and Mrs Jessie Beauge from Ohio realised that sending their son by parcel post to his grandmother would be cheaper than buying a rail ticket! They paid 15 cents for the stamps and 'insured' the boy for $50. Luckily the postmaster quickly outlawed the process.
