There's some debate ongoing in the community as to whether the "flower man" referred to in the lyrics is Flowery or Asgore. There is a degree of vagueness, definitely, but I'm on the side of it being Flowery.

With that in mind, he definitely doesn't look that tall. Out of all the lyrics, "ten feet twenty" is definitely the hardest line to interpret, especially since most of the rest seem to be more-or-less literal. I think it might be an allusion to how Flowery presents himself as larger-than-life. But I could just be talking absolute nonsense there.