The belt appears to be marked A-787. That seems to me to mean the belt is an A section (4L) and 787mm length. Since it is an A section, the nominal length would be 31" or an A31. If you're looking at 4L sizing, then it would be a 4L-33.
Put a mark on the belt and put a starting mark on the bench. Roll the belt along the surface until the mark rotates around and make another mark. Observe the distance.
If the belt is broken in half, its easy to measure length.
The outside length will translate to a 4L belt since that's the way they are sized. For an industrial A section, subtract two inches because its the center of the belt that picks up the length measurement.
Since that belt is of Asian origin, it may or may not conform to standards. It is marked as a variable speed belt but that may only be the wrappings. The widest part should be close to 1/2". If the jointer was bought used, then who knows what a previous owner decided to do about the belt.
All open stand Delta jointers are not the same so that descriptor is too vague. A stock number such as 37-190, 37-220, etc. would be helpful.
The 37-190 uses belt stock number 1340179