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On this website, they are saying that, when using a gel electrophoresis to assess DNA quality, :

High quality genomic DNA should give a major band of 10-20 kb on the gel.

Why is that so? I thought it could be because of the size of the chromosomes, but chromosome X in humans is 156,040,895 bp, which is far from 10-20kb. So why is that threshold of 10-20kb? Is DNA breaking usually in chunks with that size?

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The website specifically talks about DNA samples intended for sequencing. The DNA in these samples was/is/will be extracted from cells and this extraction procedure leads to fragmentation (breaking into chunks). Since the expected average chunk length will greatly depend on the extraction process, I guess its reasonable to assume that 10-20kb can be expected from the most common protocols.

Much larger or much smaller fragments will also cause problems with the sequencing. (This is again method dependent, but generally true with varying fragments sizes that can be sequenced with the different available methods)

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