Now, I would like to import the .xls file
Regardless of formatting, this is a very bad idea.
Basically:
Recommendation for clean working with LibreOffice when different Office programs are used.
Always create and save your files in LibreOffice and save them in ODF format (ODT, ODS, etc.).
Always keep these files as their source.
If you need other formats for distribution to partners, you can open an ODF file and save and distribute another format with ″ Save as… ″.
This way, you always have working files available in your system environment.
See:
Edit different file formats in LibreOffice
…all rows, columns and cells will be converted to Ubuntu font size 16.
Now your requirements have expanded.
A little bit longer way:
The method shown above for a document template was a bit quick and dirty, sorry.
A new document template with styles:
Open an empty sheet.
Select the whole sheet (Ctrl+A).
Delete the direct formatting (Ctrl+M).
Create a new style/cell style with the font and size you want. Cell Styles in Calc
Select the entire sheet (Ctrl+A).
Double-click on the new cell style.
Save the document as a document template.
I can’t find an “Import” option
Unfortunately, Calc is not equipped with it.
But there is a little trick to transfer cell styless to another document.
Open an empty document that has the new cell style applied to it.
And open your old document that you want to format.
In the empty document (from the template) copy an empty cell (Ctrl+C).
In the document to be reformatted, first select everything (Ctrl+A) and delete any direct formatting (Ctrl+M).
Now paste the empty cell at a suitable cell (Ctrl+V).
You have now transferred the cell style.
Select everything again (Ctrl+A) and double-click on the new cell style.
Now your document should have the desired font and size.