The size of the vice is dictated by two factors:
1. What is the Vertical Spindle to Table Clearance. (how much room is free to have a bit or a tool between the spindle and the vice, your vertical workspace).
and
2. How long of a vice can you put on the table without it sticking out the back side (and interfering with Y travel towards the Column, AND without it sticking out too far forward and interfere with the Wheel that adjusts your Y position.
You are in luck, Precision Matthews usually lists optional Vices (which they believe may fit for you on that model of Mill).
On the PM-30mv, they show four and five inch vices, to include with the swivel base... so those sizes are probably a safe choice.
The length of the table is important (33 inches in your case), but how far it can travel is also important to determine how long of a cut you can do (23 inches in your case). The length of the X axis travel is normally not a decision factor in a vice.