Bigger knives aren't always better but they can be more manageable and consistent at short/moderate distance throwing.
When I was doing knife throwing on a serious basis, I was using 9.5" cheap Hibben look-alikes with the already-poor edge filed off for half-turn and 1.5-turn (one of these forgotten in a pocket was also the cause of my expulsion from high school!).
For one-turn in particular and small competition, I used the McEvoy 79s (I think that was the model number), which were a little over a foot overall. I didn't use them too often because the knives had a tendency to ding each other up badly (moreso than the more robust shorter ones I threw), further, for beyond two-turn...I felt these were hefty and I didn't particularly like throwing them by the too-wide blade. For me I think an 11" or 12" somewhat narrower knife is more ideal for all-around throwing. The really really big ones throw well too--especially when you can watch them spin easily in flight--but I never found them to be "better"; in my case I actually found them uncomfortable after a few hundred throws.
Also when I was into it, competitions usually had rules. I think the IKTA and AKTA both had length standards for knife throwing. Of course my information is several years out of date so I would suggest looking at some knife throwing resource websites to get more concrete info. Some people like huge bowies and some people like short spikes. I am more into the spikes-side of things, but try some out and see what you're comfy with!
Sorry for the ramble!