Probably a 54.x" belt
Mower belts are often silly fractions not exact inches like car fan belts .
Post the Grasshopper part number and some one might be able to find a cross refference for you .
Is the tension arm free to swivel, did you grease it and is the spring in the right place.
I find it very hard to understand that if a 54" belt can not be fitted over the pulleys at a 55 would slip if the tensioning system was working.