True, and after doing a tiny bit of research apparently Cloud's sword is about five to six feet, and Sephy's is about 6 to 8 and I'm assuming that it varies depending on the project. So given that variable, Sakurai could shoot for the lowest, being 6 or if he wants to make it definitively longer than Cloud's being 7.
I only said Sakurai referred to Dissidia for research. And that was primarily in regards to Cloud's character design. Sakurai based Cloud's design off of Dissidia artwork, as stated in the interview with him and Nomura.
I have no idea about the moveset. But if you look at it, Dissidia Cloud and SSB4 Cloud share very similar moves because they're Cloud's moves. Finishing Touch, Climhazard, The little wind shockwave move, his side B multislash move, AND the Limit Break. It's all pretty much the same.
I'm saying that because Sephiroth would GET such delicate treatment is what would make him more difficult to implement than, say, Geno. Yes, Sephiroth is popular, but Ridley was an incredibly high fan demanded character. Sakurai compromised and made it work due to that demand. He would have to do the same with Sephiroth, but I don't think that would be his preferred option is all I'm saying. And I don't think Sephiroth, even with his popularity, has enough sway to make Sakurai prefer him over Geno. Especially because Cloud is arguably kind of representing FF as a whole. You arguably only see one rep from a 3rd party franchise outside of echoes and I think it's that way for a reason. I don't think Sephiroth would be a better sleeper hit than Geno, but he'd probably be a hit nonetheless.
I don't think Dissidia is all he uses, but he does use it. That's a fact. Going off of using Cloud's dissidia design for Smash, if Sephiroth were to be implemented, I'm only assuming he'd do the same. Looking at the Dissidia art...thats a long freaking sword. lmao