It's as easy as that. For example, the trampoline in the image above has 8 net poles.
Span a tape measure from the centre to centre of the long side net poles as shown in the picture above.
Span a tape measure from the centre to centre of the short side net poles as shown in the picture above.
Span a tape measure from the centre of the frame tube to the top of the net pole as shown in the picture above.