Hi,
99 pixels would be the height, not the width. The enforced maximum height is 92px for SD and 138px for HD mode.
The width can be up to 1280px for HD mode, and 720 px for SD mode. Ideally, the logo should not be so wide that interferes with the breadcrumb text, however, if it must, you can set the color of your text so that it is readable if it happens to be long enough to cover the logo. The logo is typically a .PNG with transparency to allow the overhang slice to show through, although you can just create one overhang slice graphic for SD and HD modes and leave the Logo out.
Note that you will need SD and HD versions of all these graphics. If you make the overhang Slice a .jpg, you may be able to achieve smaller file size graphics while retaining good visual quality. The logo, if it is to be partially transparant must be a .png or a .gif. If you can make it look good as a PNG8 instead of a .PNG24, you can again have a reduced file size, as it is important that your channel have as small a file-size footprint as possible.
- Joel