You can use this function, for example. Change for your needs and put on yours theme function.php file.
add_image_size('Special', 545, 365, true);
add_image_size('Mobile', 291, 197, true);
add_image_size('Ipad', 217, 147, true);
add_image_size('Block', 253, 172, true);
add_image_size('Single', 602, 358, true);
add_image_size('Small', 75, 49, true);
add_image_size('Block-height', 253, 245, true);
add_image_size('Related', 183, 124, true);
add_image_size('Big_one', 393, 266, true);
On frontend, inside on loop just call for example:
<?php the_post_thumbnail('Mobile'); ?>
On existing images you need to reupload, recheck. This function is accepted on new upload images.