This is a really tricky question to answer. As a toy he's Fort Max's counterpart and therefore should be on a par with him - but Phil is right, in narrative terms his character doesn't match the scale of a titan. These two have always been an odd fit - Metroplex and Trypticon are city sized brawlers in robot mode and their characters match, but Max and Scorp have always been portrayed as leaders who interact with the other robots face to face - as a scientist it makes no sense for Scorp to be the size of a mountain in robot mode.
My instinct says as a toy he should be titan sized to balance Max, but I would much rather see him as a Leader. It's just at that scale he doesn't really need the city mode. I say Leader. Then in display terms I can swap out Takara Max for the Toyworld one and claim mass shifting to get them the same height.
I'm not sure there is a 100% right answer, I can see arguments both ways for Leader and Titan.
But in my heart of hearts I think that Scorponok should be smaller than Max, like the original toy, and that it probably should be the same size as PM OP who he frequently was paired with in the comic. So Leader for me.
Scorp is Powermaster Optimus' rival. In fiction he was a city sized giant for all of less than five minutes. However every other use of him has presented him as a somewhat larger than normal sized decepticon. Leader is perfect. You get everything most people would want: robot, scorpion, base (not me. TFs becoming bases is kinda stupid, I'd rather a spaceship).
Plus getting him as a leader now, in no way prevents a headmaster titanclass in a few years.
While I can understand that most people will want him to be Titan sized (and it would be a very fun toy) as a fan of the Marvel Scorponok, I'd love a leader or Voyager version in that style. Why we haven't had either in Titans Returns baffles me.
Asuming that we are talking about a Skorponok toy using the Titans Retutn technology in another line - because they've not got long before the Titans Return product identity and packaging is retired - then:
whatever size Hasbro and Tomy believes makes economic sense. A deluxe is possibly the cheaper to make but a Titan or Leader might represent a higher return on any given nun er of sales. It doesn't really bother me because, firstly I'd probably only buy one at a TFN if someone had it cheap, and secondly scale inconsistencies don't really bother me, (Even if we had any means of working out what scale a given Skorpornok toy was anyway, given a lack of definite specific information about the dimensiona he is supposed to have in the fiction anyway)