My horse is 17.2 and a chunky ID type - big shoulders, very deep chested. I have a variety of Weatherbeeta rugs, none of which fit particularly well. The 7'3" Landa is a lovely length, but a bit "gapey" at the front, so all the rain pours down his neck. It's not very deep on him either, so gives an attractive "mini-skirt" effect! The smaller sizes either rub at the shoulders or leave his bum hanging out - or both! - and yep, all miniskirts!

I recently just bought an Orican combo on recommendation, after having sworn not to buy W/B's for him. Again it's a good length - almost a bit too long, but doesn't have enought depth for him and is too narrow through the shoulders. They are nice enough rugs and reasonably priced, but if you have a broad shouldered or very deep chested horse, perhaps you could see if anyone on your yard could let you try one out for size, as in my experience, they just don't fit.
Similar problem with the Amigo l/w - nice enough rug, but the biggest size they do is 7'0" (we can often just get away with!), and it came a bit small - not too bad length-wise, but not nearly deep enough and too narrow through the shoulders. I also found I had to let the surcingles out as far as they could go, and only JUST had enough room to fasten them!
I really like Premier rugs (
http://www.premierequineinternational.com/index.html) and Fals (the older style ones, simply don't have the cash for the posher ones!), as I know they will fit bigger horses. This year my horse has spent most of the time in his Premier l/w extra - bargain at £50! It fits him really well, and has reflective strips everywhere so I can track him down in the field at night. I think the Fals are possibly a little more hard wearing, but they are also a little more pricey! Popping away some pennies for a new t/o at the moment - going for this:
http://www.equestrianclearance.com/product/fal_pro_taska_turnout_rug_3_styles/clearance/1/index.html
He'll just have to do with the tatty old ones for the rest of the winter though!