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).
I’ve decided to send you the latest version of Ikebana (3.2, not released yet) with the latest fixes and features, along with the instructions on how to update the theme, on the same email address we used to converse previously. Please update the theme and let me know if any of the above issues still persist!