I've not found that 6x is a "plus" size - my DD is very thin, but the 7's tend to be too long and way too big in the waist. So eh hem its not because she's think lol and yes, my dander got a little up on that one... guess that's a mom for yah!
For pants I'm in looooove with the stretchy waist bands... With pants look for 7 slims if she tends to be slender...
A friend (Mom of Girls) informed me that for some reason girls size don't go 5, 6, 7 (that would make too much sense), instead its 5, 6, 6x, 7... don't ask me... but its been very true for my DD8.