Keyword exports contain single letters, two-character fragments and stray tokens. None of them are terms anyone would build a page for, and all of them inflate the list.
Setting a floor
Removing anything under about four characters clears most of the noise from a typical export. The exact figure depends on the market — some product codes and brand names are genuinely short.
Check what you remove before committing
Run the inverse filter and read what would be discarded. If it contains real brand or model names, the threshold is too high and needs lowering rather than accepting the loss.
Short is not the same as bad
A two-word term is short and often excellent. This filter is about character length, which catches fragments; judging a term's worth is a separate question answered by volume and intent, not by length.
Try it: Remove Lines Shorter Than on SeoWolf's Notepad.