In Momentum, click on the gear icon top right and click SETTINGS

  

Click on PACKAGE SIZES on the left. 


Under "New Package Size" on the right, enter the name you want to call the package (i.e. UPS Letter) and the dimensions. 


Press Save. 


Please Close and re-open the app. You will need to repeat this on the various installs since package sizes do not transfer to each installation https://shipleap.freshdesk.com/a/solutions/articles/48001080226