Regarding the single engine taxi: it's ATR's original SOP to start eng no1 during taxiing out and shut it down during taxiing in, but we don't do that anymore. Miniscule fuel and time savings are not worth the extra workload and if BTC and GCUs work less than optimally, one may lose normal braking after shutting down an engine.

What ATR calls condition levers (CL) are actually prop levers with aditional fuel shut-off function, rendering classic CLs unnecessary and reducing number of levers mounted on throtle quadrant. On old ATRs (42-300 and 72-200) equipped with electronic control unit/propeller control unit (ECU/PCU) CLs set prop RPM between 77 and 100% (100% is 1200 rpm) but only in flight. On ground ECU acts on HMU so to maintain minimum of 70.8% rpm when blades are fully flat. When you push PLs forward, PCU maintains 70.8% rpm (unless you push them very far forward, that is) but blade pitch starts to coarsen, giving forward thrust. You can move CLs anywhere between max and min rpm when on ground and it will make no difference. Trouble can develop when you set PL to take off position with CLs not set to max rpm - overtorque is guaranteed (luckilly I did it only once and it was in sim) and that's why CLs are always on max rpm for taxiing.

Surely the reason no.1 PL is bigger than no.2 PL, is so the Captain can say to the FO "mine's bigger than yours"





I'll get me coat

Finally Explained
 

Got an explanation from an engineer and seems to make sense. He reckons it is to do with the syncophaser and acts to highlight which is the master engine and which is the slaved one. The master being the number 1 engine on the ATR and hence the number 1 power lever handle being wider.
Now surely some training captain will be impressed with that piece of trivia late one night in the local over a few beers :ok: