Conversion of size is English saddle is approximately 2" larger than a western saddle.
So, you sit in a 18" English puts you in a 16" western.
Now, just like English, western seats can and will tweak that size for comfort.
Different discipline cuts in seat also shall fit your anatomy different manufacturer to manufacturer.
As was told, there are no industry guidelines for sizing, fit or trees in the saddles.
You will find just sitting in many saddles will start to make sense that certain manufacturers are more comfortable, better "feel" for communication with the horse...sounds like English saddles.
Depending upon the real width of your horses, there is also draft sized saddles with appropriate trees to fit the massive shoulders and angle.
Also gaited saddles and Arabian offer slightly different sizing and angles which changes fit to the horse.
As for clothing...comfy jeans or riding pants/jeans with a stretch weave in the fabric.
A heeled shoe/boot with minimal grip to the sole...no lug work boots.
If you have paddock boots, they absolutely work regardless of saddle style.
In time if you decide to stay with western, they spending the $ for western style clothing is great.
Till that period of time, comfy jeans, and a heeled boot to keep you safe and properly foot-bed supported is all you need.
Enjoy the new challenge.