We heard that the image logos that you could add to the header often weren't big enough, so we decided to fix that. Now the logo can be 3 times bigger in width than it was before.
Still, bear in mind that the logo size can be limited by the block height and the neighboring columns. If you resized the logo to the maximum and the size is still the same, try adjusting the columns on the sides to give the logo more space.
More on logos in the article Adding a logo.