belts by & large do not stretch, they get thinner so go loose in the V .
So for starters try measuring tomorrow he the circumference of the belt that is the belt size.
Generally craftsman have a sticker under the hood listing the belts & blades.
On mowers with manual blade engagement he exact length & width of the belt is vital as it the right kind of belt.
They must be kevlar fibre belts that are wrapped