Custom Logo in PinBlue Theme

Hi, I am a beginner and this is my first website. I am using the PinBlue theme and I want to insert a custom logo. I have been using the “Theme Logo” plugin which works but It the logo is the wrong size and looks weird. I can’t find out what exact size the logo needs to be. If someone can tell me what size it needs to be that would be great. Also my logo is a PNG file. I read somewhere that some browsers the PNG would not look good and that I should use a GIF. Is this true should I change it to GIF?

Also if there is another way to insert this custom logo and it automatically resizes to the size it needs to be that would be cool.

Thanks.