sizes appear small to me too. My 9 yo granddaughter is of average height and is already wearing size 14 tops. She's barely into 10 bottoms waist wise but they are on the short side. I think that's why short is in style. :)
I bought her a xs t-shirt in one of the teen stores in the mall. It was appropriate in style and fabric. She's outgrown that one.
I suggest that you do look in teen stores and be selective for tops at least. This may be difficult if she likes to choose her own clothes, like my granddaughter does. I plan to buy 3-4 acceptable tops, to take to her, so that she can choose 1 or 2.
It is definitely OK to have short leggings, they're called capris or ?, which suggests that you can still buy in the girl's department. EXCEPT I'm finding that even in the girl's department styles are inappropriate for a girl who is as old as 14. It seems that, so far, Target mostly has short shorts even below size 6X.
If your daughter is also larger around as well as extra tall then Sears does have sizes designed specifically for this body type.
Check out mail order sites for Sears, Penney's, Macy's. I think that they may have a larger selection than local stores who are catering to the teens and pre-teens who are doing their own shopping and/or are able to manipulate their parents. Also, I've found that some items such as knit bike shorts don't come out until later in the summer. Since the weather is cooler in Seattle they may not come out at all. Just a guess.
I have found several pairs of jeans with a natural waist line in thrift stores. And my granddaughter has been willing to wear boys jeans but she's starting to object. They run longer than the girl's jeans.