I'm wondering--do we have a rule for what counts as a novella vs. a short story? The question mainly matters for whether we italicize or put the title in quotation marks--I think that's our rule, anyway.
Mostly I see novellas described as being between 17,500 and 40,000 words, though sometimes you see 20,000 as the lower bound, and sometimes it's 10,000. In round numbers, that's like 60, 70 or 35 pages as the minimum.
It might be worth taking a look at our page on Lovecraft's word counts:
These are the stories that would be novellas with the shorter count:
| The Shunned House | 10,742 | 1923 |
| The Thing on the Doorstep | 10,954 | 1933 |
| The Call of Cthulhu | 11,905 | 1926 |
| Herbert West--Reanimator | 11,998 | 1921 |
| The Colour Out of Space | 12,457 | 1927 |
| The Dreams in the Witch House | 14,789 | 1932 |
The Dunwich Horror is the only story between 17,500 and 20,000 words.
These are the Lovecraft stories that are pretty clearly novellas--between 20,000 and 40,000 words:
| The Shadow Out of Time | 25,323 | 1934 |
| The Whisperer in Darkness | 26,624 | 1930 |
| The Shadow Over Innsmouth | 27,026 | 1931 |
Which leaves these three as Lovecraft's (short) novels:
| At the Mountains of Madness | 40,881 | 1931 |
| The Dream-Quest of Unknown Kadath | 42,589 | 1927 |
| The Case of Charles Dexter Ward | 51,112 | 1927 |
I guess I'm thinking we should go with 17,500 as the cutoff, but it's pretty arbitrary whatever number we pick.