Can I import sizing table from excel or google sheets?

There are 2 ways this can be done.

1. Upload a CSV file

See this example video that walks through uploading a csv file to create size charts

When you click on add new size chart, the following pop-up have an option to upload CSV file. 

2. Copy & paste into table editor
What you can do is open copy the table cells from excel then paste them into the app's table editor.

For example, open excel, then select the cells you want to copy. Then copy (control-c on Windows).

Then in the app, click edit on a table layout to open the table editor. Click on the top left cell in the table.

Then paste (control-v on Windows)

Then the table cells should be pasted in the app!