Well... I started at #245 and a size 22w... I have lost 49 pounds and now fit a 16 regular, or maybe 18 regular, depending on the style/brand. I find skirts are a little more forgiving than pants :)
Really, you just have to try some things on. Find something you like, grab it in 3 different sizes. That's pretty normal, to try multiple sizes of the same item, even for people who know their size! Because as mentioned, how things fit can vary a lot between designers.
I was SUPER nervous the first time I went into a clothing store that didn't carry Plus sizes... ("what if there's nothing here that I can fit! what if i can't fit their biggest size! what if the clerks know by looking at me that I'm too big for their store!" etc.). Prior to attempting this, I tested the waters at a store that carries both plus and regular sizes under the same roof. Started by trying on smaller-sized "Plus" clothing, and once I had a 16w fitting properly, asked the clerk if "do you suppose an 18 regular would be better?" and tried it on. And yes, it fit. 16w is very close to 18 regular (just a slightly roomier cut under the arms, basically).
Goal 1 - break 200 (46 pounds lost)**DONE**
Goal 2 - leave obesity behind (BMI 29.9, at 185#) **DONE**
Goal 3 - BMI = Normal (154# or less)
| current weight: 172.5