It seems that you are having some issues in order to adjust the Slingbox resolution.
If you are using the SlingPlayer software, I recommend you to stop using it and use the web based application through www.slingbox.com instead since we do not provide further support for the SlingPlayer software.
With this in mind, I recommend you to go to the Video Quality option at the bottom of the page and adjust the settings according to your needs. As a final advice, I recommend you to keep on using the Component cable, in order to get the best video signal quality.
If you do not know how to locate this feature, I recommend you to check this link.