Approximately 6 mm. It will not fit inside the 7mm (OD) tube, and operate going up and down.
So, any modification will have to start with replacing the 7mm design with some other size. Now, why were we modifying, again????? Start with a "Parker design" kit!!
"We" weren't modifying anything, but "we" like everything about the Euro kits except the fact that they use Cross refills. It has always seemed to me that Parker refills wrote better under less-than-ideal conditions than does Cross. The Cigar seems most like the Euro in style, but is enough bigger that it is awkward to carry in a shirt pocket. I prefer to turn a comfort-bump just above the nib, and therefore don't like to write with a Sierra or Atlas. I was looking for a reasonable alternative.