One of the arguments available for the shortcode is “size_large”. It allows you to specify the image size to be used in the slideshow by referencing the image size name (i.e. medium, large, full). However, it does not accept pixel dimensions in the current release.
Probably the best solution would be to add a custom image size to your theme’s functions.php file by using the add_image_size function. If you give that custom size the name “slideshow”, then you would reference it like so in the gallery shortcode:
[gallery size_large=slideshow]
You will likely need to regenerate image thumbnails after adding your custom sized image in order for it to be available when called. The Regenerate Thumbnails plugin by Viper007Bond makes this a trivial task.
Another option is for you to redefine the Large size image dimensions in your WordPress Media Settings to be 1500 (max width) by 1500 (max height). Of course you can set the height to some unlikely value such as 99999 so that even vertical oriented images will be 1500 pixels wide.
Let us know if you have any further questions or if this solves the issue. Thanks.
Thank you very much sir!
add_image_size method working perfectly!
