Resizing thumbnails

I’m probably an idiot, but i’ve read all the above topics about size of thumbnail etc. but I can’t figure it out. It just automatically sets to 150×150. And i heard you told someone about the add_image_size function in funtions.php, but i’m not sure where to clearify that in the following function.

<?php if( function_exists( 'easy_image_gallery' ) ) {
    echo easy_image_gallery();
		}

	remove_filter( 'the_content', 'easy_image_gallery_append_to_content' );  ?>

I hope you can help me.

Kind regards Manon

https://wordpress.org/plugins/easy-image-gallery/