Okazaki fragments are formed during replication of the lagging DNA strand. What determines the length of these fragments?
The length of Okazaki fragments is not necessarily a tight distribution. The lengths are determined by the spacing between adjacent sites where DNA primase has synthesized a short RNA primer on the lagging strand at an active DNA replication fork. In E. coli, as I recall, this occurs on an average of once per 1000 nt.
DNA polymerase holoenzyme then uses that 3'-OH on the RNA primer as a substrate and synthesizes the DNA strand complementary to the Template strand UNTIL it bumps into the next RNA primer.
It is those nicks in the newly synthesized lagging strand, that have a deoxy-3'-OH, and a ribo-5'-phosphate, (that need to be removed and ligated) that mark the boundaries of individual Okazaki fragments.