These lengths are bundled into hanks for despatch. The length of about 1.4 metres allows for two shorter lengths for those happy to avoid tangles and thread the needle more often. The full 1.4m length is probably about as much as can be managed without too many tangles whilst sewing. "25" is a guide to the thickness of the thread, and "3" is the number of separate strands spun together to make it. Most threads are three stranded. (35/3 is regarded as thin, 18/3 as thick)
25/3 is a good thread to use for larger books, especially those bound codex style. Linen thread is very strong, and for most bookbinding purposes a thinner thread will have more than sufficient strength, and will cause less swell. This thread is waxed in the factory after manufacture.