As most everyone has said, there is no optimum. But you should always have a good, reliable fixed blade, 3-3 1/2 inches or longer. If you have survival skills, that blade should be able to do for you until the calvary arrives.
Otherwise, it's a question of intended use. If you know beforehand what specific tasks lie ahead, you can equip yourself accordingly. In general, three blades will cover most anything in the wilds (the northern wilds, at least). A multibladed folder like an SAK or a Leatherman, a good fixed blade (my current preference being something like the Gerber Yari, BM Nimravus, ot flat-ground Grohmann Camper or Boat Knife, (but I would also be very happy with a Frosts Swedish Army knife, especially for whittling), and an axe, 3/4 size or larger. A good bucksaw would also be handy. The axe and saw will be handy if large amounts of firewood are needed or for instance, say you are traveling by canoe in late fall in the northcountry and you take a spill. If it's been raining, it may be hard to find the wood you need to get a fire going to warm you up and dry you out. A good axe will be the quickest way to save your hide because you can use it to split up standing dead wood very quickly. And in such situations, time could be a major factor before hypothermia sets in. Just make sure the axe is firmly lashed in so you can recover it.
There are a lot of variations to this and I'm sure you could work the big chopping knife in there somewhere. If I know I'm going to be cutting a lot of saplings (such as creating shooting lanes or clearing trails), then the big knife will always get the nod. But if you are lugging a big knife around just because you think it will save you, you could be in for a surprise.