Roughly it will be SL * SMG *1.8.
Fold it in half and measure across. = SMG = spinaker mid girth or .9 * J
Measure from the head
to the tack rings. = SL = spinaker luff length or I
Here's ours 1.8*26 (J) *86 (I) = 4024 sq ft
I'm not sure our assy is full size, it may be slightly smaller. The sailmaker
may have used a 1.65 multiplyer but I've never checked or cared. The 3/4 sym kites are probably full size but again never checked. The 1.5 and 2.2 sym kites are surely smaller but again I've never checked. We don't race
enough anymore for me to worry about it.
Curious, why do you care as long as it fits your fore triangle? Measure the SL and see if you can hoist it. Even if it's over by a few feet it won't matter unless you're racing
and need it to measure in.