Spec for Site Logo in GDF Modern

Hi,

I use GDF Modern Child and am looking at replacing my site title and tagline with a Site Logo in GDF Header options; I will be putting together a logo that includes text – basically, the existing Title and tagline with a logo on the left of the text. I have made basic logos as .png files before using LibreOffice Impress (open-source version of powerpoint) – e.g., https://loopyfood.net/marketing-materials/. Very basic – I am not a trained designer, but I am happy with these so far!

Now that I want to make a Site Logo, what size image should I be aiming for (optimal/maximum), and would a .png be ok, or should I have a different format (simple logo like above, with some text). If possible I will use Impress again – or do you recommend me using something like Gimp?, which I will then have to learn!

Thanks