It looks like Article 1 Section 2 has what you're looking for, specifically "The Number of Representatives shall not exceed one for every thirty Thousand, but each State shall have at Least one Representative" [1]
Wikipedia also seems to provide a good summary of the issue. [2]
[1] https://www.archives.gov/founding-docs/constitution-transcri...
[2] https://en.wikipedia.org/wiki/United_States_congressional_ap...