Example of fpw_image_size filter in Feature a Page Widget 2.0. By default, Feature a Page Widget registers three sizes, one for each layout: fpw_big, fpw_square, fpw_banner. NOTE: To produce consistent image sizes and a good widget layout, CSS rules like max-width may at times affect the image's apparent size on the front end.

fpw_image_size_filter_examples.php

<?php
/**
 * replace one of the widget image sizes
 * 
 * examples replaces the banner layout's image with the 'large' size image from Settings > Media
 * 
 * If you replacing $size with a custom image size, you must register that separately with add_image_size()
 * 
 * @param	$size	string		slug of registered image size
 * @return	string|array	slug of registered image size or "array( width, height )" (not recommended)
 */
function fpw_change_image_size( $size ) {
	// check if image uses the "banner" layout image size
	if( $size === 'fpw_banner' ) {
		$size = 'large';
	}
	return $size;
}
add_filter( 'fpw_image_size', 'fpw_change_image_size' );