Jennifer,
 
We have similar issues at our site, which includes outdoor programs, house tours and rides on a horse drawn railroad.
 
We have set tour size limits for each part of the program.  The limits are constantly "pushed" by adding a few extra guests.
 
Tours that are too large make it hard on the docents/guidesm, they result in damage inside the house, and the reduce the quality of the visitor experiance.    We base the tour size on; size of indoor space (your local fire marshall can give you a formula based on square feet of a room) on seats available (on our train) but also on the size of the group a docent can handle.   We also take into account class size (all of our programs can accomidate one class, some can handle two)
 
The preasure to allow tour group size to grow comes not only from within but also externally...  I once had an asian woman tell me "we are chinnese, we are small" while pleading for a tour twice our normal size... (in her case she was trying not to hire a second interpeter)
 
Good luck, it will be an ongoing battle.
 
Randy Hees
Patterson House at Ardenwood Farm
City of Fremont, California
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