Cannot adjust Label format for Avery Zwreckform Sheets

When attempting to use labels, I have entered the brand as Avery zwreckform from the list and specified the size of the individual blank labels, but under format, I am unable to specify the number of columns as three and the number of rows as seven. The figures merely revert to one. Also the page width seems always to be 27.35cm, no matter how often I change it to 21 cm.
Each Avery Zwreckform sheet has 21 labels of 6.35cm x 3.81 cm ie. three columns with seven rows as above. How can I adjust the settings, as the sheet always shows just one label in the top left hand page, instead of 21.

Thanks in advance of any help that you may be able to give me.