[Koko Ed]

Small Regional (24-35 teams)
Pros Intimate. Easy to scout. Almost garunteed to get into tournament if your robot functions reasonably well. Intimate. Alot more matches.
cons Quality of competition is thinnned out. A good chance of slim picking for second allaince pick. Very fast turn around time (little time to make adjustments or repairs). Smaller venue.

Medium sized regional (35-50 teams)
Pros Qualitly of competition may improve due to a higher amount of teams particularly in the playoffs. A little more difficult to scout but not all that much if your scouting team is on the ball.
Cons Quanity doesn't always equate to quality. Less matches. Not as intimate. Not quite as likely to make tournament.

Large regional (50+)
Pros Larger field could mean better competition especially in the tournament. Larger venues. Greater prestige from comming out on top of a larger field (?)
Cons Can be overwhelming. Tend to not be very intimate. Not as likely to get into playoffs unless your robot is at top form. Difficult to scout (unless you have a top notch scouting team). Once again quanity does not always equate to quality.