A minimum length of 2 inches for clippings, please. The longer the coat before trimming, the more yarn you will get, and the added length will increase the yarn’s sturdiness.
Please let your groomer know that you need the trimmings as long as possible, and ask them not to collect the shorter buzzed stuff. If the clippings are too short, it will either be returned or discarded, or evaluated for the possibility of blending (very short buzzed fibers will be rejected, as they represent a lung risk for long-term inhalation–the super short stuff really poofs into the air while I work and floats around the room. Wearing a mask would combat this for me, but not protect my family. Such short clippings behave differently than shed fur–clippings can result in fibers that are a lot shorter, and the way I work with the hair gives it more opportunity to become airborne en masse).
Except from soft-coated non-shedding dogs and felines, clippings tend to result in a pricklier, and sometimes less sturdy, yarn. The decreased durability comes from the presence of slick, straight guard hairs, which can make the yarn easier to pull apart when tugged. How well clippings work for yarn depends upon the slickness of the coat, how straight the fibers are, how soft they are, and how long they are.
In general, don’t shave your dog or cat to collect their fur, unless you have a non-shedding breed, or a long-furred cat that you shave anyway. If you already shave your dog for summer, the fiber can still be useable, although you’d probably want to wear a liner for whatever hats or mittens you’d make with the yarn. It’ll still be very warm.
Clippings from long haired cats are better than clippings from shedding dogs, because cats’ guard hairs, as well as undercoats, are softer than dogs’. The prickle-factor isn’t an issue with cat clippings.