In order to decrease the gallery size in a single portfolio view, please add this to the Custom CSS area in Dashboard > Vamtam > General:
.portfolio-image-wrapper {
max-width: 700px;
margin: auto;
}
Adjust the bold number.
Here's a guide how to add the code:
http://support.vamtam.com/solution/articles/200025-how-to-add-custom-css-code-