Most labels do correspond to standard sizes, of which Word has a huge range available, however if you do have a non-standard size, you can create it Mailings > Labels > Options > New label - Give it a name in place of the name already listed, add or change the measurements and save it. It will then be available to use in mail merge. It helps to start from a label that is close to what you have already.

If you are creating a custom label, then create a new document from the label and add borders to the cells. Then print it on plain paper which you can hold over the label sheet to check the dimensions rather than waste lots of labels getting the emasurements correct.

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Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com