I found Masta rugs come up larger than Weatherbeeta if that is any help. My boy is normally in 6ft rugs but his 5'9" masta fits him well.
If your horse measures for eg. 5'10", I would go with a 5'9" masta rather than the size up.
Not sure, but I put my 4 year old's masta h/w stable rug on last night and it's too short on him since the last time he had it on! Gorgeous rug, so I'm gutted I'll have to go and get another one a size bigger!
I havent tried shires but i find generally mastas are built for bigger built horses
I also think it depends which rug you buy. I have a horse who is a 6'6 in their check teck lw turnout and i bought a 6'6 in one of their heavy weight designs and its slightly on the small size but it is well padded, maybe it will get bigger when the padding goes down a bit!