Mine have to have 5 ft exercise sheet but there arn't many makes that make them that big ( mark tood and rambo are the two makes i have), so depending how chunky yours is you may get away with a 4 ft 9 ...
my boy wears 6'6" turnouts, and 4'3" exersize sheets fit him perfectly, not too big or too small, so maybe a 4'6" would be ok, i always prefer exersize sheets to be on the smaller size so they dont flap off their bottoms
Ive got 2 exercise sheets (one flourecent one not) for my 16.2 WBxTB at 4"9, fits perfect, but wouldnt hurt to have another inch or two, so I fould imagine you would maybe need 5ft.