Calc: How to set default font for new CSV files?

I’ve successfully edited the default style to use a large font when I create a new calc document. However, when I open a CSV file, the font is still tiny.

How do I specify the style / font size when opening a CSV file?

Interesting question. Since the default template is not used on the opened CSV file, it appears that styles need to be set manually (F11) for this new spreadsheet.

See this comment in an old big report, 86336 – FILEOPEN Calc ignores custom template when opening csv file

Nevertheless, you can apply a different template (your default one) to an open document if you use the Template Changer Extension. This is referred to in the Calc Guide, download from English documentation | LibreOffice Documentation - LibreOffice User Guides, under Changing the template… in Chapter 5

Tested with
Version: 25.8.3.2 (X86_64)
Build ID: 8ca8d55c161d602844f5428fa4b58097424e324e
CPU threads: 8; OS: Windows 11 X86_64 (build 26100); UI render: Skia/Raster; VCL: win
Locale: en-NZ (en_NZ); UI: en-GB
Calc: CL threaded