I can't remember the last time I load-tested a constant voltage system (70.7 or 25) and found the speaker load to be equal to the amp's power rating.
Design specs usually call for 20% headroom, and many times I see 50%. That means a 100 watt amp shouldn't have more than 50 speakers (each tapped at one watt) connected to it.
A 100 watt amp may have 100 speakers, each tapped at one watt, but they're only 3dB SPL down in each coverage area if you tap them at 1/2 watt.
The system now has headroom, the amp's running cooler, and you don't have to deal with line loss. You can also expand the system easily by just adding speakers. If you need that extra 3dB of loudness, move up to a 200 watt amp and tap each speaker at 1 watt.
Keep in mind, too, that although a system is called a "70.7-volt constant-voltage system" it will rarely, if ever, reach its rated voltage during normal use. There's some more built-in headroom. (But it needs to test at full load during the sign-off stage.)
Wire size? Typically 14-gauge trunk lines from amp(s) to the field, 16-gauge zone drops from the trunk to each section of the building, and 18-gauge speaker lines within the zones.
(Er ... Have you seen the price of copper lately?!? The thinner the cheaper!)
For added protection against failure, high-pass the entire system at 100Hz and don't boost the bass on any mixer amp or EQ.