By waist cincher, I'm assuming you mean the non lacing up kind? Like a mini girdle? If it's that kind, then the fit is the same as a girdle as the elastic in them will stretch.
I remember you saying your waist was 25" so you would fit looking at a girdle/cincher with about a 22" waist depending on how much you want holding in. My waist is 33"- 34" and in girdles I take a 30"- 31" one. 29" is too much of a struggle to get on without the help of lacing like corset does.
If you buy a girdle in your waist size (25") then it will just sit on you like a shirt
If it's the waist cincher corset kind which just controls your waist and nothing else, then the fit is the same as it would be for a normal corset, about 4" or more depending on your desired waist reduction and body type.
With corset cinchers, they are easier to tighten too as they just work on the stomach and lower rib area with is quiote fleshy and lower ribs are bendy. I can corset alot tighter in a cincher than a full corset as you don't have the resistance of your upper ribs and your hip bones. Of course too, the larger you are in the waist, the easier it is to acheive a dramatic waist reduction as fat squishes. Leaner ladies have less fat so bone and muscle doesn't squish as much.
Those with broader ribs will take a larger corset size than those with smaller ribs too. My ribs are quite narrow so I can get away with wearing smaller corset sizes (24" and 26") than most other size 16's can wear whereas, one of my friends who is only 1" shorter than me but the same dress size has to wear a 34" corset as she is alot larger boned than me.
Officially, you should leave at least a 2" gap at the back of your corset to allow for fat and skinny days and to allow for "spring" (movement of the corset for comfort) BUT because corset waist cinchers are often worn as belts, they are often worn closed. I can close my cincher so I should probably move down to a 24" waist but sometimes it's nice not to have a gap at the back if I'm waring it outside of my clothes.
You have the choice with a cincher but it shouldn't feel loose on you, it should feel like you are getting a great big bear hug.
Chris, from your photos and your waist size, I think that you have small ribs too, so if its a lace up cincher you are after, then you should look in the 18"- 20" range depending on how adventurous you are are if you want to close it. If you want to close it, then you will probably be ok in the 20" one once you get used to it. With a 22" corset cincher, you'll be able to close it too quickly and it will feel "too big" after a while.
Lei