Full Width Grid Responsive Picture Resizing

Is there an easy way to do that?

No, I don’t think there’s an easy way to do that, since the change you’re proposing would require some substantial modifications. The theme is designed to responsively adapt to screen size and reduce the number of squares in each row accordingly. Five squares across wouldn’t be legible on an iPhone in portrait view, for example.

The default size of the homepage boxes is set in functions.php:

add_image_size( 'pictorico-home', '590', '590', true );

But the display at smaller screen sizes is controlled through media queries in the stylesheet.

You can learn more about using media queries that target certain screen sizes here:

http://en.support.wordpress.com/custom-design/custom-css-media-queries/
http://css-tricks.com/snippets/css/media-queries-for-standard-devices/
http://webdesignerwall.com/tutorials/responsive-design-with-css3-media-queries