Assuming your fly fishing as you're asking about tippet. Leader length is determined by conditions and what type of fishing you're doing. Casting dry's 9ft is considered on the short side so fish aren't spooked, longer than 15ft is getting on the long-side for most of us to cast and turn-over. The darker the day and more coloured the water allows a shorter leader vs clear water and bright day often requires as long as you can manage. For wet fly and streamer fishing with a sinking line you can go much shorter - 7-9 ft with nymphs below an indicator or classic wets swung to as short as 4 ft for large, heavy streamers that you'll be swinging and stripping for the big browns of the Cowie.
Tippet size, not strength, is determined by fly size - too heavy and the fly doesn't float or drift naturally, too light and the leader won't turn the fly over. Simple rule of thumb is to divide the fly size by 3 and go up or down from there depending on conditions. E.g a size 4 leech or sculpin pattern for browns you can use a 1x tippet but a size 16 elk hair caddis dry you'll want 5x. Strength of the x depends on the quality of the tippet you buy and whether you use nylon mono or fluorocarbon. Most quality brands have a much higher breaking strength for the diameter size than yesteryear. Rio's fluoroflex, for e.g. is 12lb for 2x, 7lb for 4x and 3.6lb for 6x. for 3x and below your knot strength is going to be more important than the tippet rating as a 6lb rating can be half or less with a bad knot. Learn a good no-slip loop knot, use lots of saliva and you should be golden!
Cheers!
Ukee
