Unfortunately you’re confined to the three sizes in masonry (large, medium, small).
However, you can target those three classes and give them custom sizes:
.masonry .grid-sizer,
.masonry .masonry-post {
width: 33.333%;
}
.masonry .masonry-post.width4,
.masonry .grid-sizer.width4 {
width: 66.666%;
}
.masonry .masonry-post.width6,
.masonry .grid-sizer.width6 {
width: 100%;
}