16.1 WB 6'9" - she can squeeze into a 6'6" but the larger size accomodates her on her fat days and with the amount she eats she'll probably have lots of those!
my very long welshy d gelding is about 15.2/3 and takes a 6'6 rug but some of these can be quite a tight fit!. Lucky for me though as all my previous horse rugs were 6'6 and 1 6'3 for my 16hh + horses!