Recommend size of logo is 220px (width) X 35px (height). This works with most of the themes.
If you want to know the exact size of logo used in theme that you’ve installed for your site, open the front end site and then right click on the sample logo that is shown and save it to your machine. Open the logo using any image editor in your machine and it will reveal the exact dimensions of the logo. Use those dimensions for your own logo.