Both my faceplates are 3 inches in diameter. I glue (titebond II) two 3/4 inch pieces of hard, non oily wood like cherry together. I then bandsaw them round about 1/2 inch bigger then the faceplate. then I screw the cherry to the faceplate, put on lathe and turn it close to faceplate size. Then I glue whatever bowl blank, or segmented bowl to the cherry with (Titebond II) turn the bowl and then part it off the cherry block when done. When you part it off you can waste the cherry glue block saving the bowl blank. Then you true up the cherry and it is ready to go for next one. I have turned bowls as large as 12 inches in diameter with this method. Remember this is only for dry wood with a smooth glue joint. Never glue on green wood. Green wood should only be put in a chuck or turned between centers.
If I was going to do a large diameter, very tall vessel I think I would use a larger faceplate, but for bowls and short items the 3 inch works fine. Also if you were to do tall vessels you would want to use a steady rest
The worm screw should hold whatever piece the lathe will handle. I say this because you should
ALWAYS have the tail stock touching the wood when using the worm screw. Think of the worm screw as you would the regular drive center.
Always bring your tail stock up to wood whenever you can no matter how you chuck it up on the lathe. Also never do anything on the lathe you are not comfortable doing.
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I hope this all made sense.