This letter appears to be on European stationary, if you measure it's aspect ratio. Either that, or cut-down Legal. It's not the standard US Letter size.
One clue was contained in the missive to the New York Post:
The letter, which read in part, “Death to America,”
wasn’t printed on a paper size normally found in the US,
says an FBI official familiar with the matter.
An FBI spokesman declined to elaborate.
Erich Speckin, who runs a private forensic laboratory in Okemos, Mich.,
says the heigh to width ratio was 1.4 to 1, according to a photo he saw.
He says that ratio is common in Europe and elsewhere, but rare in the US.
That could suggest that the mailer may be from another country
or may have travelled outside the US.
This letter appears to be on European stationary, if you measure it's aspect ratio. Either that, or cut-down Legal. It's not the standard US Letter size.
Okay, here's a better picture. This is clearly European, A4 format.