@GregK: There is no option to change to 175×175. But if you upload the Featured Image of this size then it will show 175×175. But if you upload the larger size then it will crop into exact 210×210.
But if you want to make in 175×175 as default even when you upload the large image, then you need to build child theme and then copy content.php
file from simple-catch theme to your child theme and replace the code:
<?php the_post_thumbnail( 'featured' ); ?>
With the following code:
<?php the_post_thumbnail( 'thumbnail' ); ?>
Then activate your child theme and go to “Settings => Media” form your WordPress Dashboard and then in “Thumbnail size” type in width 175 and height 175, select “Crop thumbnail to exact dimensions (normally thumbnails are proportional)” and Save Changes. After this you can use “Regenerate Thumbnails Plugin” and regenerate thumbnail which will change your thumbnail image in all your old post.