Flank cinches come in a standard length. The width and general shape depends on what you're doing. As with your girth, the wider the surface area, the better the pressure distribution is and the more comfortable your horse will be.
Personally, I use a 30" string type roper girth on the front (This one
), and an 8" roper type flank strap (This exact one
).... on a barrel saddle with slanted roper stirrups... because we trail ride and this seems to be the best set up for the horse I have.
What kind of pad are you riding under the saddle? How thick? It sounds like your saddle may be too wide in the gullet for your horse if it's genuinely sliding back and forth when stopping or climbing/descending a hill.
Have any pictures of your horse saddled up?
And yes, a properly fitting western saddle, with the right type of pad under it, shouldn't slide back and forth or rock a whole lot. It will still have a little movement to it which can chafe or rub a horse though, and that's why a western saddle is designed to use a flank strap. Do you absolutely need one? That's a topic that gets debated a lot. Depends on what you're doing.
In an arena, riding flat ground? Don't really need one. Out riding trails, or roping or barrel racing, spending a day checking cattle or riding fences? You need one.