The volunteer ‘Readers’ were instructed to write out the words andsentences on small 4 x 6-inch pieces of paper, known as ‘slips’.
Volunteer 'Readers' for the Oxford English Dictionary were encouraged to write down interesting headwords along with their appearances in-situ along with the associated bibliographical information. The recommended paper size was 4 x 6-inch pieces of paper which were commonly called 'slips'.
(Double check this against the historical requests from James Murray.)