Replying to Back protector (red impact waist coat?)

hi guys,because i do not like the idea that i will be sitting on a wheel chair for the rest of my life after some stupid fall,im thinking about some back protector...i have chosen the red on.now the question - how exactly should back protector fit?where on my back it should start and end? i know i should try it but in my local shop they do have only medium size that fits me quite well but im not sure if it is long enough and i cant try on large...or some nice sizing chart for red will be great.i cant find it anywhere. thanks a lot