Bucas Flysheet Fitting

I recently went to purchese the Bucas Buzz-Off Flysheet for my pony who is typically a 69". The sheet was way too big and almost went down to her knees. I have heard that Bucas usually runs a big large like Horseware but I have several Horseware blankets in 69" and they fit perfectly. The blanket did have a buckle flaw so I’m not sure if the sizing was a flaw as well or not. What size would you recommend I get now? Should I try the largest pony size or the smallest horse size? Thanks