Big; there's two issues, the first being sawdust, the coarse stuff that falls to the floor, and the extremely fine particles which float in the air for a long time.
The larger particles are simply a housekeeping problem. The finest particles, say from one micron up, are the serious health risk.
So you're actually dealing with two separate problems. Air volume and air velocity are the two factors which address both of the created dust types which the dust collector is trying to collect.
If you box in the underside of the TS you still MUST allow make-up air into the enclosure; that's what picks up the particles and carries them into the piping.
simply enclosing the saw cabinet and sticking the DC pipe (of whatever size) will accomplish nothing. This is not a vacuum cleaner.
Having the dust pickup in the bottom of the tablesaw cabinet will collect a LOT of both types of dust, but unfortunately the spinning blade will carry lots of the finest particles around and kick them into the air. So collecting from both sources is ideal.
The bigger the dust collector, the more air volume it will draw and consequently a larger pipe diameter is needed to feed that air requirement.
The relationship between Velocity and Volume is a matter of physics. There are tables (calculations) for this stuff. A 1 1/2" hose is useless for a tablesaw. 4" with the appropriate Hp and proper sized impeller is the minimum for effective dust control.
Sure, a lot of members have fairly decent shopvac and separator systems that seem to work reasonably well, but I'm pretty sure most would admit that given the space and the cash, they'd gladly pop for a 2HP piped system. Me included; mine's a 1HP DC and it just isn't enough.