According to a recent news report, this is the longest stamp ever issued for Sri Lanka (123mm L x 35mm W):

If anyone's interested in the entire news article it is at this link:
http://www.sundayobserver.lk/2011/02/06/new30.aspMaybe this a question for topical specialists, but are there any other countries that have issued longer stamps?
I imagine the Sri Lanka stamp would look great on a cover!