We use these leads ourselves and have done for several years, we love them for there all round ability.
The length of lead is a personal choice but usually the smaller the dog the longer the lead. The most popular length of slip lead is 1.5m (5') in 8mm and 10mm, and 1.2m (4') and 1.8m (6') in 12mm.
What diameter (thickness) lead will I need?
8mm - The most popular size of rope or braid lead. Strong enough for all breeds and a nice size to handle. A perfect pocket size lead, whether it's a clip or slip. These leads are used and recommended by dog trainers and behaviourist.
10mm - This size of lead is for the stronger dog and kind to your hands. Our 10mm 1.5m length matt slip lead with a leather stop is a popular lead. Our 10mm 1.2m length is popular for a clip lead.
12mm - For the larger dog, a good chunky strong rope lead, popular for both slip and clip leads. Slip leads are popular in lengths 1.2m and 1.7m with or without stops and for clip leads, the 12mm is popular in 0.65m and 1.2m lengths.