This article from DiviBooster has helped me resizing the image size. Following is the modified copy of the CSS with the values which I have used in my website.

.et_pb_fullwidth_header {
padding: 0 !important;
}
#copyright{ visibility: hidden; }

/* Set the image widths */
.et_pb_gallery_grid .et_pb_gallery_item,
.et_pb_gallery_grid .column_width,
.et_pb_gallery_grid .et_pb_gallery_image,
.et_pb_gallery_grid .et_pb_gallery_image.portrait img
{
width: 330px !important;
}
.et_pb_gallery_grid .et_pb_gallery_image img
{
min-width: 330px;
}

/* Set the spacing between images */
.et_pb_gallery_grid .gutter_width { width: 25px; }
.et_pb_gallery_grid .et_pb_gallery_item { margin-bottom:25px !important; }
/*——————————————-*/
.et_pb_gallery_grid .et_pb_gallery_item { clear:none !important; }
.et_pb_gallery_grid .et_pb_gallery_item:nth-child(3n) { margin-right:0 !important; }
.et_pb_gallery_grid .et_pb_gallery_item:nth-child(3n+1) { clear:both !important; }
.et_pb_gallery_grid .et_pb_gallery_item { margin-right:24px !important; }
.et_pb_gallery_grid .et_pb_gallery_image img { min-height: 0 !important; }
.et_pb_gallery_grid .et_pb_gallery_image,
.et_pb_gallery_grid .et_pb_gallery_image.landscape img
{
height: auto !important;
}