I use size 2 on the front and 3 on the back and my 16.2hh idxtb. not sure how much bone she has but she has a decent amount. They are more than long enough plus wide enough!
I also use then sometimes on my sisters 16hh welshies who has alot of bone and they fit him better than the ww ones he has!
I like the shires brushing boots. They are just soft neoprene with the hard bit in the middle but they are so soft and fit well and do the job. They only cost me £8 off Ebay, but that was over a year ago, but they have lasted. I have 2 pairs now. I like the soft ones as there is more support. I dont like the hard ones like the ww ones for that reason. I have 4 pairs of hardish boots that have hardly been worn at the yard. My husband did buy me a pair of the trizone ones for Xmas, but I havent tried them on yet. They feel soft but on the picture they seem to stick out not round the leg if you know what I mean.