Here's a good question I got today.
Is there such thing as a band that's too big?Yes there most certainly is. One easy way to know if you've gone a bit too far- if the stage will have to be bigger than the dancefloor to fit the band...it's too big. If you want a true live band experience, I vote for nothing less than 3 pieces. After that, another piece per 20-25 (starting with 3 pieces and 75 guests)people is a good rule of thumb.
Some things to keep in mind though:
Typically what makes that big band sound is a horn section. And that usually requires a 5 to 6 piece band.
The other element that makes a huge difference in not only the sound, but also the playlist- having a female vocalist.