It depends on your throwing style. I have a wall full of throwers, mostly balanced for spin throwing. I don't use any of them any more, since I throw no spin and decided to make my own to match my style. I agree that a heavy knife is better than a light one as far as being forgiving of bad throws. The heaviest knife we ever made was an offset, 310 grams, 10.5". It really ate up targets--penetrated to deep. Now, the range of knife weight that I use is between 250 (light) and 305 (heavy) grams. That's just me and I'm still learning, like most of us!
As far as length, my favorite design is 9" long. Unfortunately, it is not IKTHOF compliant for no spin! So now, I just make all of mine to be around 10.5" with a BP of 39% and a minimum of 25 grams/inch to avoid loud ringing when I occasionally (less often these days) slap the target sideways. There are many good knives for spin, but not too many balanced for no spin. If you're really good, it doesn't matter. For the rest of us . . .
So, back to my two cents worth: it depends on your throwing style, also your level of experience in throwing. Make your own, if you can! If you can't, there's some pretty good advice in this forum.