It depends on the template you are using.
In our newer templates “Verde” “Aquarius” and “Cherry Blossom” the max size for the logo is 180x130px.
In our “Electrify Black” template the size for the logo is: 210x 120px.
In our “Win 8 style” template the size for the logo is: 175x146px.
And in our “French Lilac” template the size for the logo is: 180x180px.
In all templates, if you do add a larger image the image will be proportionally resized to fit these dimensions.
Feel free to comment here if you have any other questions.