The clay I buy comes in 25-lb bags. So I can make five 5-lb plates from one bagful.
As I am not a tall, brawny person, and I left age 30 behind some time ago, I look for shortcuts that will save wear and tear on the various parts of the body that work hardest in making pottery on the wheel, namely shoulders, lower back, neck and wrists. The following is a great way to make bigger plates more easily.
Rather than get to work centering a 5-lb lump of clay, then pushing it down by manual force into plate position on the wheel head, I used a punch-down technique that was easier.
First, I put the big ball of clay roughly in the center of the wheelhead. I got the wheel rotating slowly. Starting from the center and working my way out to the edge, I thumped my fist on the clay in a rhythmic pattern as it revolved. In this way, I covered a 13" round circular batt (a board) on the wheelhead with a relatively even layer of clay about 1 3/4" thick. It was WAY less work than the regular raising and lowering when I center a bigger ball of clay.
Then, using water and pressure as I would normally, I "threw" the battful of pre-flattened clay, from the middle of the clay out to the edge of the batt. In essence, I was smoothing flat the fist-marks from the thumping that I used to spread the clay on the batt. The smoothing process brought clay out to overhang the edge of the batt. I easily cut off the uneven edge with a needle tool, which centered the mass (1-2-3!) into a truly circular disk. Voila, centering, with very little work!
Then I threw an indentation in the center. Raising, lifting and thinning the clay at the outer edge of the batt, it was simple to create a rim on the plate. The rim grew to extend past the edge of the batt. In this way in relatively short order I threw five plates of the same (13.5") diameter.
(The angle of the photo distorts them a little- they are actually all round and all 13 1/2" in diameter.)
Just FYI, after the porcelain lost enough moisture content to be firm and trimmable, and I was able to trim away clay from the foot, I weighed the 5-lb plates. They weighed between 2 lbs 8 oz and 3 lbs 2 oz. When they are fired, meaning all water weight will be gone, they will weigh less.
I do know now that it really helps to have an electric wheel! And it helps greatly to have Youtube videos to "teach an old dog new tricks".