ContractorTools does not allow you to set the size of your company logo, but it displays it as large as possible based on the resolution and height and width of the image that you provide.
The space that we allocate for the logo in the letterhead of our PDF documents has an aspect ratio of 2.6:1 (width:height), so if you want your logo to appear as large as possible, you should crop the logo to fit that aspect ratio (width x height).
For example, the logo could be 260 pixels x 100 pixels, or 520 x 200, or 1040 x 400 etc. The larger the image (the higher the resolution), the larger it can be displayed. So it's best to provide an image of 520 pixels x 200 pixels or larger.