How long is a daddy longlegs?

The most common use of the expression daddy longlegs is for a crane fly which is an insect, with six legs.

It has 6 legs and 2 antennae that look like legs so that is why everybody thinks that daddy long legs have 8 legs.

Daddy longlegs is also used, in North America, the UK, Australia and New Zealand for some types of spider and harvestman, both of which have eight legs. So this question does not have a simple answer.