Trophy Trophy 1200 1998 rear shock length

I think your best bet would be to stick your bike on the centre stand and then scrabble about underneath it with a tape measure - though preferably not the one out of SWMBO's sewing basket!
But if you have a death wish or a craving for hospital food .......... .

 

I would suggest getting a YSS shock.
Not many bikes have the large hole at the bottom for the pin to fit. Infact I have never heard of any replacements. The YSS offers good adjustability and has 10mm of length that can be altered. I have a Maxton on my Trident but have fitted a few YSS and all are very happy with the purchase. Plus in my opinion is better built than a Hagon for similar cost.

 

If it helps, and it probably won't, I went down the Hagon route for my Trident. I can't say how it performs against other makes, but they do set them up for your weight, riding style and useage (solo, 2 up, panniers etc). And it is much better than the original. Compression and rebound damping are not adjustable separately though, but then again the basic Nitron rear shock that I had on my Daytona 955i CE was the same, and more expensive, though possibly better quality.
The Hagon is rebuildable with (in my case) a same day turnaround.
It all boils down to it being your money and your choice. But could a shock designed/set up for a lighter weight sports bike be overwhelmed somewhat by a heavier bike? I genuinely don't know the answer to that one (I could guess!), but I would be looking for something from a similar sized bike if I went down that route.
Nowt's easy is it??

 

I would speak with the technical guys at Hagon, YSS or any of the other suppliers you may like, they are very knowledgeable and more than helpful.
Rider weight, regular pillion, luggage, style of riding, type of roads, they have it all covered.
It is worth spending a bit of time before buying.
Hagon were great for me and my Bonnie, but it was a fairly basic solution for my requirements.