Thank you
Usually the module automatically splits your gallery in to columns which can be setting in the module settings. It is this setting that defines the width of each image.
To define your own static pixel width you can add the following to the Custom CSS field of your templates parameters (Extensions -> Templates -> [YourTemplateStyle] -> Custom CSS)....
Edit to taste..
.flexgallery .j51flexgallery {
width: 300px;
}
Ciarán