Try this method, although I'm not sure it's what you ultimately want.

Create and customise your calendar in Publisher in the usual way and at the default calendar size.
Now print to PDF or Export as PDF.

Locate the saved PDF calendar and open it in a PDF editor/reader/viewer that is capable of scaling/fitting the print size to a larger printer paper size of your choice.
My example below uses Adobe Acrobat Pro DC but Adobe Acrobat Reader DC (free) has similar options as will other modern PDF readers.
When open in the reader/viewer, opt to Print and then change the paper size, type of paper, quality, check orientation etc.
In the Page Sizing and Handling options, use the Fit option which up-scales the smaller size to the new larger paper size.
Print when printer is loaded with correct size paper.
Note: Paper sizes available will be in a range that your printer can use.
Click the image below to see a short video of the above.
My basic calendar was created in Publisher 2016 at A4 size and up-scaled to A3 via printer paper option and PDF editor/reader option to fit.
You could save your creation as a Template should you wish to for future use.