Ah, I see now. The back-end allows for two sizes (“Normal” and “Large”), mostly for aesthetic purposes and to preserve the theme’s visual concept. In order to add a third size, several theme files should be customized, namely:
– functions.php (add a new size choice option for project item editor mode).
– custom-styles.php (add the new size in pixels and well as link to the Theme Options hook).
– header-functions.php (adjust the column width in the Isotope settings to match the new dimensions.
Since this functionality is not envisaged by the theme, the standard ThemeForest support offer does not cover such deep customization; unfortunately, I am not available for a paid ad-hoc project at the moment, but I can advise a website with decent value-for-money in terms of good-quality WordPress customization services: Tweaky.com.
Sincerely,
Andrew / Satori Studio