World's smallest ad was commissioned by Guinness World Records for the launch of its site in 2000, was worn on the knee band of a bee and measured 100 x 100 microns. The previous title holder was an advert measuring 17mm by 12mm. It was a half-page ad in the world's smallest newspaper, an edition of Brazilian weekly Vossa Senhoria published in 1999 (source: BBC).
The largest advertising poster measured 274 m (898 ft 11 in) long, was made in 1998 by Saatchi & Saatchi to promote Pucini's Tosca and set up in Rome (view it here). The title is now being contested by Mazda, who unveiled a 1,020 ft-long poster in January.
A train traveling between cities in South African Guateng province is the world's largest moving billboard, measuring 268.25 m (880-ft 1-in). It was launched in 1998. This title is also disputed, by Qatar Airwaves that launched an aerial light show in South Africa's largest cities in January.
World's largest billboard appears to be the one at UK's Fort Dunlop. It was erected in 2002, weighed about two tons, and was 132 meters long and 23.7 meters high. The previous title holder, 300 meters long and 45 meters high, was built in China's Chongqing municipality in 1998. It was surrounded by fog the year around failing to attract a single advertiser and was set to be dismantled in 2002.
Finally, the largest neon advertising sign was made in Hong Kong in 1999 of 800,000 light bulbs.