Can I change the default size of a new table in macOS Pages?
You have a number of options available to you, such as creating a template, using AppleScript or Automator to create applications, scripts, or services, and so on.

Below is a sample script I've created based on the details in your question:

tell application "Pages"
set myDoc to make new document
tell page 1 of myDoc to set myTable to make new table with properties {column count:5, row count:2}
tell cell range of myTable
    set width of columns to 80
    set text wrap to true
    set alignment to left

end tell
end tell


Assuming you're not familiar with how to use this, below is one way you could use the above script:

Create a service using Automator


Launch Automator (usually found within your Applications folder)
Go to File > New
Select Service and click the Choose button (Note: If the 'Service' option is not available, select Quick Action instead)
In the top right hand of the window that appears, ensure that "No input" is selected from the Service receives drop-down list
Ensure the In drop-down list is showing "Any application"
In the second column containing the long list, find "Run AppleScript" and double-click on it
This will add the Run AppleScript window on the right
Now, replace the (* Your script goes here *) with the the sample script above 
Save the service using a meaningful name (e.g. Create Pages Table).


Now you should be able to run the service from any application by going to the Services list within any Application menu (e.g. Pages > Services, Finder > Services, etc) and select the service you just created. 

NOTES: 


The set width, set text wrap and set alignment lines within the script are not required. I have included these so you know how to customise these table characteristics if you need to.
I have assumed from your question that your two by five table referred to five columns by two rows. However, you can safely change the column count and row count values in the script to suit your requirements.