Yes, you can print on A5 size paper in QuickBooks Online, Sales.

I would be glad to guide you through the process of accomplishing it.

In QuickBooks Online (QBO), you can choose the paper size you prefer when printing invoices and other sales transactions.

Here's how:

1. From the Invoice window, click Print or Preview.
   
   
2. Click on the Print button.
3. Click the Print icon again at the top.
   
   
4. Select More settings.
5. Choose A5 from the Paper size drop-down menu.
6. Click Print to proceed with printing your documents.
   
   

Additionally, you can adjust more on the settings in the Print page to ensure proper display and printing of your invoices or sales forms.

Furthermore, you can check this article to learn about printing or emailing numerous sales forms: Email or print multiple sales forms in QuickBooks Online.

You can always post again whenever you have further concerns about printing or other issues you may encounter.