Here’s why that’s a harder question than you think:
Antares makes a variety of seat configurations, and each rides a little differently. For example, they make a flat seat, a medium-deep seat, and a deep seat. So a 17" Antares with these different seat options would offer different amounts of real estate for your behind. Incidentally, Antares also makes seats of different widths, from roughly 26 cm across to 28 cm across. So even two 17" Antares saddles with a medium-deep seat could feel different if they had different seat widths.
I could show you 5 different riders who are 5’6" and 120 pounds, and depending on the relative lengths of their upper and lower legs, their torsos, and the shape and size of their behinds, they might need a huge variety of seat sizes: 17", 17.5", even 18" or 18.5" in some cases. It has more to do with your body proportions than your weight, and it often varies from saddle to saddle. You might be a 17" in this one, a 17.5" in that one, and an 18" in that one. And the news gets worse: some of this comes down to personal preference. Some people like to ride a little “small” and others like to ride a little “big.”
I will tell you this much, though…the average H/J rider in the United States is riding in tack that’s arguably too small for them, and they don’t even realize it. Also, MANY riders need to size up 1/2" or more in a cushy high-end French saddle brand like Antares, Devoucoux, etc. So if I were a betting woman, and I were accounting for your 5’6" height, I’d say you likely need a 17.5". That’s a common “correct” size for women of that height. But again, this is sort of like saying “I’m 5’6” and 120 pounds, but you’ve never seen my body. What size jeans do you think I should I buy?"
Also, anybody know of anyone selling used Antares, or a site for them? I’ve looked at www.iselltack.com and the site off of Antares for used saddles and they seem to have nothing there for me. I’ve also looked on Ebay and found one for $1,800 but chickened out because it was a 2006 and I wanted to see what else was out there.
Sure, there’s plenty of vendors who specialize in high-end used French tack. fineusedsaddles.com, highendusedsaddles.com, marylandtackexchange.com, used-devoucoux.com, and used.cwdsellier.com are just some of them. Edited to add: allthebestusedsaddles.com, which is Beval’s used saddle site. There’s also some vendors who don’t specialize in French tack but tend to have some of it in inventory, like olsonstack.com and middleburgtack.com. (Do yourself a favor and skip Danforth Fine Used Saddles, which I haven’t listed here because of rotten customer service stories from multiple buyers.) And of course, there’s lots of places to look for privately sold Antares saddles, but your odds of getting a trial or return policy are much lower with a private seller.
Lastly if I use a half back pad (I already always do), will that make the saddle fit better to most/all horses?
Let me put it this way. If you ask 20 different women with different feet/shoe sizes to try on a particular size 7 shoe, and you made all of these women wear really thick wool socks when they try on the shoe, would the thick socks make the size 7 shoe a better fit for every single one of these women?
You can probably see where I’m going with this. The answer is, “The thick wool socks will probably help the fit for some women whose feet were just a little too narrow or short for the size 7 shoe. But it will make the fit way too snug or uncomfortable for other women whose feet fit comfortably in the shoe before they added the wool socks. And the socks will do nothing either way for women with gigantic or teeny feet because no amount of padding would make their foot big enough or small enough for a size 7 shoe.”
Now swap out the words “thick wool socks” for “half pad” and “women” for “horses,” and you’ve got your answer about half pads. If in doubt, learn a bit about saddle fit so you can appraise whether a half pad is helping or hurting the fit on any given horse. A site like this will get you started, and it includes some basic info about fitting saddles to riders as well:
Also if I were to get a saddle and looking to have it fit primarily warmbloods, would you say I should go up a tree size? Is medium decent?
There’s an anal-retentive way to answer this question and a non-anal-retentive way. The anal-retentive way is to say “Warmbloods, like all horses, vary enough in their back structures that no single saddle could fit all–or even most–of them.” The less anal-retentive answer is that many horseless riders and trainers, especially in the H/J world, manage to achieve an acceptable fit on many warmbloods using a saddle with a medium-wide tree.
The trouble is knowing whether you’re looking at a medium-wide tree. Many French saddle dealers try to suggest tree width by offering a “dot to dot” measurement across the front of the saddle’s gullet. Again, there’s an anal-retentive rant to be made against this measurement, which for many reasons is a ridiculously rough and approximate guideline to the point of being borderline useless. But the non-anal-retentive answer is that most horseless riders/trainers are looking for a dot-to-dot measurement of roughly 4.5 inches.