If you are making a new invoice and want to make it in A5 format, follow this process:

In your Dashboard, go to Invoices, click on "+New". In your New invoice add all the necessary details - client, payments terms, items, transport labels etc, then click on "Preview & save".

On your right, in Invoice Options, select the A5 format, as seen below:

If you have an existing invoice that you wish to print as an A5, go to Invoices, click once on the invoice that you wish to change, click on "Edit" at the top of the invoice, then click on "Preview & Save" and from Invoice Options select A5 and Print.