I happened to find this excerpt from the novel online (my paperback is packed away somewhere in the attic):
"There was a clown in the stormdrain. The light was far from good, but it was good enough so that <PERSON> was sure of what he was seeing. It was a clown, like in the circus or on TV. In fact he looked like a cross between <PERSON> and <PERSON>, who talked by honking his (or was it her?-George Denbrough was never really sure of the gender) horn on Howdy Doody Saturday mornings-<PERSON> was just about the only one who could understand <PERSON>, and that always cracked <PERSON> up. The face of the clown in the stormdrain was white, there were funny tufts of red hair on either side of his bald head, there was a big clown-smile painted over his mouth. 417
If <PERSON> would have been inhabiting a later year, he would have surely thought of <PERSON> before <PERSON> or <PERSON>. The clown held a bunch of balloons, all colors, like gorgeous ripe fruit in one hand. In the other he held <PERSON>'s newspaper boat..."-(IT Novel)
"Then the clown’s face changed. And what little <PERSON> saw next was terrible enough to make his worst imaginings of the things in the basement look like sweet, angelic dreams; what he saw destroyed his sanity in one clawing stroke. And <PERSON> knew no more..." -(IT Novel)
As you can see, the imagery is mostly left to the imagination with a bit of generic "clown description" thrown in. Which reminds me of an old <PERSON> quote (he, the former frontman of Rock pioneers <PERSON>); "Nothing I say will ever be as sick and twisted as what your dirty little minds will think up!" <PERSON> novels tend to use this same philosophy, where he will generically lay out a base and usually fill it in with "The terror of which can never be comprehended" or whatever, so that you create the ugliness without him having to spell it out.
And so, neither the upcoming movie's version of Pennywise nor the upcoming miniseries will really match his looks, and until they're actually released we won't know whether or not his personality and mannerisms will match what we've read. 417