I would call 10mm 3/8 and I believe you are making the splice way too long. Check the directions again
http://www.samsonrope.com/site_files...Eye_Splice.pdf
These are the instructions for the fids you are using. I know some fids are half as long and you may have instructions for those fids.
The easiest way to splice is with Brian's splicing wand, btw.
http://www.briontoss.com/catalog/tools.html
Allen