John could not have said it better!
A"canoe paddle!" HA!!!
OK, seriously speaking, if it were me, I would get MORE input from what the customer wants, I like mine around 15"(OAL), give or take an inch or 2 either way, it all depends on what he will be doing with it. If its going to be a safe queen it doesn't HAVE to be huge. But, if he plans on using it, then there "should" be some prerequisites that are needed for the desired purpose he has in mind for it. I think an average, would be around 15" OAL. That's with a 5" handle, on the average. Talk to your customer, that's the main thing, he will give you more insight than anyone here can! The one that is paying you has the most impact on what the outcome should be!
This is ONLY my OPINION, and as you know they vary, everyone has one, and when it comes to selling the knives you make, ONLY 1 opinion matters. Do understand that customer( well, most customers) have no idea what they really want, it's up to you to guide them, you may have to ask a lot of questions. Such as, " what do you want to use it for" , chopping/ fine cutting, how well will they take care of it, ( this lets you know if you should use stainless), if they are old school and LOVE High Carbon steel, they won't accept anything else, so ASK!
I hope this helps, and please excuse my dumb sense of humor, for some reason I have to attempt to be funny ALL THE TIME!
Rex