Use View Size
The Use view size is used to set your output to a 1:1 (real world) scale.
The Width and Height fields are used to control the size of image as measured in pixels or specific units. Click on the chain icon to the right of the text fields to lock the aspect ratio of the export (Mac OS X).
Resolution (Mac OS X)
The Resolution drop-down list is used to select the resolution for the exported image. For viewing images on the screen, 72-96 pixels/inch is adequate. For printing, you might want to increase the resolution to 150-300 pixels/inch. Remember, high resolution images take longer to generate.
When enabled, SketchUp will smooth the exported image. This method can take longer, but it helps to reduce jagged lines and pixilated artifacts in images.
The slider bar for a JPEG export is used to define the level of JPEG compression quality. The level of quality becomes lesser and the file size is smaller as the slider is moved to the left. Conversely, the level of quality becomes higher and the file size is larger if the slider is moved to the right.
The checkbox for Transparent Background allows your image to export to have transparency for any non-geometry in the model. This options is only available for PNG and TIF file formats.