For a normal MMJ to MMJ cable, I'd suggest
starting with a 10ft cable.
That seemed to be the most versatile length and was the most popular
length that I used to sell in my eBay store. The DE9 to MMJ cables I sold
were slightly longer at 14ft, which people really seemed to like (I sold
100s of them).
I owuld gues a lot of people could solder the other end of the cable to a
DE9, DB25, or whatever, it's getting and crimping the MMJ that is the
problem. So a long-issh cable (3m or so) with an MMJ on one end only
would be useful. The purchaser cuts it to length and solders the right
conenctor on the other end.
My cheap-ish Multicomp (Farnell own-brand) crimp tool claims to do 'DEC
plugs'. I've never tried it, and have no idea which make of MMJ it likes.
It has woreked fine on every 4p4c, 6p4c, 6p6c aed 8p8c I've thrown at it
though.
-tony