A 25-foot cord is suitable for walking around a room. It's the most popular length for home phones -- particularly kitchen or workshop wall phones -- and the least popular for office phones, because it makes a mess on a desk. Un-stretched, it's about about 36 inches long.
A 12-foot cord is suitable for walking around near a desk or standing near a wall phone. It has become very popular for office phones and is often the right length for a cashier at a restaurant. It's also good if you have a king-size bed. Un-stretched, it's about 17 inches long.
A 6-foot cord is suitable for sitting at a desk or lying in bed (single, double, or queen-size) when the phone is on the night table. Un-stretched, it's about 12 inches long. It's the standard length handset cord, supplied with most phones.