Slidedeck Pro WordPress Image Size

does it have a control panel with sizing options? Is there any mention of 270px at all in the files. If not then either your image is 270px or WordPress is resizing it. If WordPress is doing it check out this page settings>media

EDIT:

// Correct image aspect ratio to fit within skin's image constraints
if( !empty( $image ) ){
if( !empty( $image['width'] ) ){
    if( $image['width'] > 500 ){
        $w_ratio = 500 / $image['width'];
        $image['width'] = floor( $w_ratio * $image['width'] );
        if( isset( $image['height'] ) ) {
            $image['height'] = floor( $w_ratio * $image['height'] );
        }
    }
}
if( !empty( $image['height'] ) ){
    if( $image['height'] > 250 ){ // Change the 250. This is the height you want so change to your height you use for your images
        $h_ratio = 250 / $image['height']; // Same here change the 250
        $image['width'] = floor( $h_ratio * $image['width'] );
        $image['height'] = floor( $h_ratio * $image['height'] );
    }
}
}

Note that if you update the plugin you will have to change this every time. It is best to ask the dev if there are any plans for a control panel or a place to assign custom sizes.