It is a fairly practical idea to use the heavier boulder-type stones at the bottom of a wall so that you don't have to lift them and so that you can bury any irregular bottom surfaces of these stones in the ground and orient the better faces outward. If you use the bigger stones first, you will then be able to determine the number of irregular contour problems which you are going to have to deal with, (and fit other stones to) as you come along the wall with the next row (course) of stones.
The biggest stones you have in your stockpile should give you and idea of the width of the new wall you are going to build. If the wall is too narrow the boulders will stick out the sides of your wall, which is okay, if you are okay with how that looks. If the wall you are going to build is only a little bit wider than your biggest stones then you will not be leaving yourself much space to build on the other side of these stones properly. It's not good to be not believing yourself too much.
I generally plan on leaving at least a foot or more on the other side of the largest stones I will be using, and so determine my initial wall width that way.
If you do have to lift big stones up onto the wall don't lift them, roll them up a plank or up a ramp of stones.
Remember too that you don't have to use every big stone you come across. Some of them will be too big or so round that they will be almost impossible to build with and no matter how much you would like to believe they could work in the wall, they may in fact, compromise the structure and completely drain you from doing much more on the wall for the rest of the day.