A pallet has many standards, which are usually Product dependent. however, due to standardising warehouse racking concerns and international containerised freight volumes, some standards do exist.
Europallet Standard. 1200mmx800mmx150mm
The Euro pallet was originally designed by the Germans to fit easily in their railcars and containers. The pallet is a 4 way entry pallet, open for Forklift access on each side. Usually constructed of softwood, these are usually destined for a one way trip, then disposed of.
More detail:
http://www.europal.net/en/Infos/enaccueil_info.shtm
Australian Standard. 1165x1165x150mm
This pallet is the standard Pool or Hire Pallet as used extensively in Australia. Constructed of Hardwood and armoured with Steel caps on the bearers, this pallet survives where others fall apart. They last for years.
Other standard sizes exist, but I have little experience with them.