I did it!
I ended up doing it 7" high and 4 1/2" wide. I ended up doing the wrapping method and what i did was roll it out into a long rectangle and then put a napkin on the top (to stop it sticking or squishing cose wasn't sure) then rolled in up onto my rolling pin and then stood it up and wrapped it around the cake smoothing as i went along.
It didn't slide down or anything even though i used buttercream, it did got a teeny bit wavy at some sides cose of the buttercream but really not too bad.
I ended up carving circles from a high sheet cake and then stacking them up, frosting between and then 1 dowel down the middle.
Here it is if you are interested, thanks for your help guys!