Know where you're coming from, my 6yo is a big tall girl as well.
Unfortunately she is very girly and the boys stuff wouldn't work for her and even worse is most of her friends are smaller than her and still fit in the age 6 clothing which is frills and fluff and I'm having to get her the least vile stuff from the older kids section.
Try shops in the US, they often have plus or husky ranges for the bigger built child. I'm finding my daughter still just fits in some of the 7x stuff from US whereas she is easily age 9/10 here.
thechildrensplace (they seem to have a good shipping offer on atm, I ordered $95 and only paid $20 shipping this morning)
I also like boden clothes, usually buy from UK, some of their things are generous in size though it pays to read the reviews
Seeing as we are opposite seasons there is still some sale stuff hanging around pretty cheap!