The short answer for a Russian Blue
Most adult Russian Blues fit a medium carrier about 19 to 20 inches long, 12 inches wide, and 12 to 13 inches tall. That size suits a cat who is medium in build but long and lean, with a dense, plush coat that makes her look a little larger than she measures. The real test is not the label on the box: inside the carrier, a Russian Blue should be able to stand without her ears brushing the roof, turn around without bumping the sides, and lie stretched out with her legs extended. A carrier that only lets her crouch or curl tightly is too small, even if she can be coaxed into it.
Why a Russian Blue measures longer than expected
A Russian Blue carries her length in a slender, muscular frame rather than in bulk. Owners who shop by weight alone often pick a compact carrier meant for a stocky cat, then find their Russian Blue cannot stretch her hind legs or lower her head comfortably. Measure her twice: once from the tip of her nose to the base of her tail, and once from the floor to the top of her head while she stands naturally. Add a few inches to the length measurement and at least a small margin above the height measurement, and use those figures against the carrier's interior dimensions, not the exterior.
Choosing the shape and door for this reserved breed
Russian Blues tend to be devoted to their own people and wary of strangers, so the carrier itself can become a source of dread long before a trip begins. A top-loading carrier, or one with both a top and a front opening, helps here: you can lower a reluctant Russian Blue in gently instead of pushing her toward a single small door while she braces against the frame. A rigid, hard-sided carrier suits this breed better than a soft-sided bag for vet visits, because it holds its shape when she shifts her weight and it gives her a solid floor underfoot. Check the latches by hand before every trip, and choose a carrier with ventilation on at least two sides so air moves through without leaving her staring out at a busy waiting room from every angle.
Getting a shy Russian Blue comfortable before travel
The best carrier size still fails if the cat panics the moment the door closes, so treat the carrier as furniture for several days before any journey. Set it in the room where your Russian Blue already likes to rest, door open, with a familiar blanket inside that smells of home rather than a freshly washed one. Let her investigate on her own terms, and feed a few treats just inside the entrance so she walks in voluntarily.
Mistakes that make a good carrier feel too small
The most common mistake is buying for a kitten's size and never upgrading, followed closely by stuffing a thick bed into a carrier that was only just big enough. A Russian Blue's dense double coat already fills space, so use a thin, grippy pad rather than a bulky cushion that raises her into the roof. Compare the interior numbers, and remember that a carrier that fits a Russian Blue for a short vet trip may feel cramped on a longer journey where she cannot stretch as freely. If she is between sizes, go up one size: extra room is forgiving, while a tight fit is not. In KinsLe, Family Sharing keeps the Russian Blue's measured carrier size, door style, and calming routine in one shared profile, so a partner or helper packing for the vet uses the same fit the cat already trusts.