The right tool for the right job. Certain people seem to get confused on this. This is another slight rant on blades. So, 3 basic sizes of knife, SMALL, MEDIUM, and LARGE.
Small knives are useful for everyday tasks like food prep and carving, and can be pushed into some chopping and splitting roles with the use of a batton. However, if you plan to do this, make sure your knife is up to the task. Best used for small tasks.
Medium knives are blades at least 5 inches in length, up to 7 or 8. These blades provide a nice balance between light duty tasks as mentioned above, and heavy duty tasks like shelter making and fire prep. Decent all round knives.
Large knives are blades such as machetes, parangs, kukris and goloks. These blades are far more efficient in chopping and splitting capability, making them far more ideal than the other two for shelter and fire prep. Depending on the size, you can actually use them for finer tasks, for example, i carved a spoon with a machete once. However, it takes practice to make this a workable option.
Best advice, If you only want to carry one knife, a medium sized blade is probably your best bet. However, I find that carrying 2, or even 3, makes things much easier, and means that you dont risk blunting/breaking blades on tasks they arent designed for, or injuring yourself trying to push a blade into a job it wasnt designed for.
Your mileage may vary. As always, practice before you are in a situation where you may need to use these items. You will be much more efficient in your work, AND you will get an idea of what system works best for you.