Cornell notes template, Legal size
The Cornell layout on Legal paper, which buys you roughly nine more lines in the note area than Letter.
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Three more inches of the part you actually use
Legal is the same width as Letter and three inches taller, and all of that extra height goes into the note area rather than the cue column or the summary. At this ruling that is around nine more lines per page. For a lecture that runs long, the practical effect is fewer page turns and fewer moments where a thought gets split across a break, which is where notes tend to go incoherent.
The cue column is the method
Cornell notes are three regions: a narrow cue column on the left, the main notes on the right, and a summary across the bottom. The method depends on filling the cue column after the lecture with questions your notes answer, and writing the summary later still. Most people who say the system did not work for them ruled the page and then used it as lined paper. The ruling is a prompt, not the technique.
Where Legal is genuinely the right call
Law and accounting still run on Legal, so if your notes are filed alongside other Legal documents the size matches. It also suits subjects with long worked examples, where a derivation that spills onto a second page is harder to check than one you can see whole. For everything else, Letter is easier to find, easier to print and easier to store.
Printing it accurately
This only measures correctly if your printer doesn't rescale the page. In the print dialog set Scale: 100%, not "Fit to page" or "Shrink oversized pages". If you're not sure it worked, print the calibration page, measure the ruler on it with a real ruler, and it will tell you exactly what your printer is doing to the page.