I used to lease a TWH - and rode her english at that, she was a high withered narrow girl, and I used a Wintec Sport (which you can hardly find these days) with a fleece pad, and it fit her well. Look at the gullet size (in inches) of your western saddle, compare the measure to an english saddle. Usually Western comes in FQHB and SQHB. Which as I see it, they have a bit more wiggle room when it comes to sizing because the pressure is spaced over a wide area, with english, its spaced at a smaller area, so its more important to get a good fit. If you are thinking about getting an english saddle just for kicks and seeing if you like the sport - go for used vs new, no need to spend the extra money if you don't think it will fit. So for some tips with buying used, my background is in saddle wheeling and dealing, and minor saddle repair. Heres my advice to a new comers guide for buying cheap saddles.
1. If you flick it with your finger, and it sounds like cardboard, Don't get it. Most cheaply made saddles are unbalanced for you and your horse creativing pressure points and made with terrible leather.
2. Go for older saddles - They were made in the time of quality not quantity.
3. But make sure everything is SOUND, simple tree checking, hold saddle cantle to your belly and pull on the other side. If it creaks or bends, its a broken tree. Pull on all the things sticking out of it, flaps, straps, ect.
4. If you are just trying it out - I wouldn't suggest buying anything for more than $100, With the horse market being down (where I am from) Saddles can be found fairly cheap.
If you are starting out - I would stick to deep seated saddles - such as a dressage saddle, they are more secure and personally find them more comfortable. All Purpose saddles can also be deep seated, and they are suitable for a variety of different work. For your sanity, I would steer clear of jumping, closecontact, or saddleseat saddles - they require more skill to stay in, and would likely discourage you right off the bat. Wait until you have practice until launching into those.
As for your position, you'll feel less secure in an english saddle, that's for sure. Its YOU holding YOU in the saddle. You'll be in more of an alert and ready for action seat, then relaxed on a trail ride.
Western saddles are about your butt size, English saddles are all about your leg size. So, to figure it you have to measure your upper leg (you can find guides online) from your hip to your knee. I would guess its around a 16. I am 5'4" and I ride comfortably in a 17, and find 18" too roomy for me.
I feel like I have gone on for days -