Chain for what? Bicycle? Dog? Chaingang?
Assuming it's for a bicycle, if you have more than one but less than 9 rear cogs, then you need a 3/32", or 8-speed chain. If you have 9 cogs, you need a 9-speed chain. If you have 10 cogs you need a 10-speed chain. If you have one cog, you need a 1/8" or singlespeed chain.
All chains are longer than you need unless you ride a recimbent bike. Take the old chain off and stretch it out next to the new chain. Remove any extra links so the new chain is the same size as the old one. If you ride a recumbent, you will probably need to buy more than one chain and put them together. If you have a tandem, you will also need to buy two chains.