Hey Timon,
you’re right, this size is a gap in the tile size assortment! I’ve added the code to include 3×1 and 1×3 masonry tiles – please download this archive, unzip it, and upload the resulting file into the theme folder (/wp-content/themes/Ikebana). Please let me know if it is functioning properly; in which case I will also include this feature into the next theme version (3.2).
Sincerely,
Andrii / Satori Studio