The axle diameter is 28mm where the bearings contact it......as for the thread, that would need a vernier to check the outer diameter of that....that said, the washer the nut runs against is 25.2 id
So your guess at 24 is probably not far off....
If anyone wants to put a set of calipers on the threads of the rear axle to verify I would appreciate it.
I believe I found info that the SV1000 uses a M25 x 1.5 rear axle nut like some other Suzukis. I sold my SV and got a GSXS that uses a single-use (distorted thread flange nut) rear axle nut that bites into the axle threads (in place of a cotter pin). I attached a pic for anyone curious.
I am looking to replace the GSXS nut with another style of rear axle security nut that I can re-use and that will not mess up the rear axle's threads. I think the SV1000 nut will work (after drilling some holes in the axle for the cotter pin).