Glad you like using the Flex pro theme. Regarding the slider image height, it is controlled via image-size explicitly defined in the functions file.
*** Backup before making changes. ***
1. Open functions.php file (Appearance > Editor)
2. Change the height 550 in the following code:
add_image_size( 'box-slider', 1080, 550, true ); add_image_size( 'full-width', 1900, 550, true );
3. After making this change, you need to upload the slider image again. Try to use a different image name or delete the existing image.
P.S. For best results upload image of exact required dimensions. For example for keeping 700px height, upload 1900 X 700 image size.