Hello Carine, favicon is the little icon that represents your site in each browser tab, you can edit it in studio -> Designer -> site icon.
As you can see in the attached example, in this case the downloaded image is 256 x 256.
The image has been resized automatically to:

• Apple Touch Devices (152x152)
• Facebook Share (100x100)
• Favicon (16x16)

So I would say at least 152 x 152 to answer your question.
Then you have the logo of the site, which you can modify in Studio -> Designer -> Logo Default size is 240 x 48.
If you do not have a logo, and "Text" = 0 then this is the title of the site that is used in Studio -> Settings -> System ->Site Stettings.
If you complete "Text" then it is he who is displayed in priority on the title site.