If one wants a 3/4" full port ball valve, does that mean the inlet & outlet ports will also be 3/4"? Or, is that not always true. For example, could the connections be flared to fit, say, 1.5" pipng?
You can do this, but it will introduce a restriction in your piping. May be an issue if you are worried about pressure drop.
You could put whatever reducers you wanted on it. As mentioned, you will get a pressure drop, but normally large valves are one line size smaller than the pipe for economy, although with valves as small as 1-1/2" and 3/4" it wouldn’t make much cost difference, and I’d probably go with full line size.
Here’s the formula you want to use:
Pressure drop in psi = (gpm)^2*specific gravity of liquid/Cv^2
The Cv, or flow coefficient is something you’ll have to get from the valve manufacturer, it’s specifically calculated for porting and trim of the valve as well as the diameter.