Rotary tables are never big enough. If they are big enough, they're too heavy to lift.
By the time you get the work on the table, and allow space for clamps, you can eat up a lot of space. So get as large a diameter as is in scale with your equipment, both in diameter and thickness, as you can. Maybe somebody else can give you a better feel for what that might be for your machine.
Horizontal/vertical is a good idea, by the way.
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