what size and format (gif,jpg, or ...)to change the logo please ?
The best logo format is .png. It has a transparent background and this should make the logo fit great on any theme.
The logo size depends. Can you let us know which theme you are using? If you want to check the size of the logo for one of our themes demo, you can do that by pressing Ctrl + Shift + C
and hover over the logo. This will show you the size of the image "box" on the website.
Check the screenshot
as an example. This is the Emporium theme demo.