Hey Irin,
first of all, please accept my apologies for such a long delay in the reply!
The width of each portfolio item is determined automatically depending on the number of items per line – they all take up equal share of the total content width. You can set the number of items per portfolio grid line using the “Number of portfolio items per line” setting on the “Portfolio Page” tab of the Theme Options panel.
Thanks for your patience!
Sincerely,
Andrii / Satori Studio