A banquet chair is about eighteen inches deep. Pull it back far
enough for a grown adult in a sari or a suit to stand up, and
you’re two feet clear of the table edge. Put another table
behind that chair and its guests want the same two feet going the
other way. Leave enough room for a server to walk between the two
chair backs carrying a full tray and you want three feet, and closer
to five if they have to pass while somebody is standing up to hug an
aunt.
Run that outward from a 60-inch round and the table stops being five
feet across. It becomes a circle roughly ten feet across, and inside
that circle sit eight people. Ten feet across is about 78 square feet
of floor, which is a shade under ten square feet each — before a
single aisle has been drawn anywhere. Once the aisles are in, the
planning figure most people in the trade work to lands between 12 and
15 square feet per seated guest. You can do it at 10. It feels like
10.
So a hundred seated guests is somewhere between 1,200 and 1,500
square feet of dining floor. That is the number before the
dance floor, the buffet run, the head table, the cake table, the DJ
and the bit of floor by the door where people stop to greet each
other. Add those back and a hundred-guest dinner with dancing wants
something closer to 2,000 square feet to sit in without anyone
turning sideways to get past.