Added option to set size of Orbit slide

You can now set size of the Orbit slide, eg
<?php OrbitSlider("slide_number: false; bullets: false","medium"); ?>
To use a different size other than thumb, medium, large or full, use the
add_image_size function in the functions.php of your child theme, eg
add_image_size( 'orbit', 1000, 300, true );