It does not matter if it's on the skinny or wide side, as long as the surface the unit mounts on is 10" wide. There may be issues if the duct were say, 3x10, but any duct large enough for the whole house will be fine with the unit installed on any surface at least 10" wide.
Almost all thermostats have a FAN switch, that can be set to ON or AUTO. In the AUTO position, the blower comes on when the furnace needs it (during heat/cool calls). In the ON position, the blower will always be on. This is usually controlled by the G terminal on the furnace and thermostat.
I'm not sure exactly what you're trying to do, so I'll cover a couple different setups
Blower on when you want
For this situation, simply connect the G terminal on the thermostat to the G terminal in the furnace. When you want the blower on, set the thermostats FAN setting to ON.
Blower on when house is too warm
If you want the blower to come on when the temperature in the house raises above a set temperature, you'll simply connect the Y terminal of the thermostat to the G terminal in the furnace. This way whenever the thermostat calls for cool, the blower fan will come on.
Depending on the furnace, you may be able to connect the Y terminal on the thermostat to the Y terminal in the furnace, and simply not connect any control wires from the furnace to the A/C unit. Most furnaces activate the blower after a predetermined (possibly programmable) delay, when the thermostat calls for cool.
Blower speed (Multi-speed blower)
Blower speed may vary based on the different functions. For example, heat might be medium low, fan might be low, while cool may be high speed. So you'll want to take that into consideration as well.
Without more information about your specific equipment, and what exactly you're trying to accomplish. It's difficult to provide more accurate, specific information.
Best Answer
The intake duct comes from the heated air supply -- so basically the closest place you can get it before any of your zones (to ensure it is getting air anytime the furnace is on, regardless of zone).
The humidistat can go anywhere it's able to sense humidity, though many have a probe designed to stick into the airflow of a duct. If duct-mounted, the normal place is to put it in the return vent before the humidifier: this gives a good overall sense of humidity levels in the house. For the most part, the house will mostly be at a uniform humidity level, but temperature can affect humidity so if there are rooms/areas of significantly different temperature they might have a different level.
Here's my setup, for reference:
I used a piece of 5" semi-rigid flex duct and a couple of adjustable elbows.
Initially I was using the humidistat shown here, but I've since switched to having it controlled by my thermostat, which automatically adjusts the humidity based on outdoor temperature to keep it as high as possible without getting condensation on the windows.
Also, if it came with a saddle valve, do yourself a favor and throw that in the trash. You'll have to turn the water on and off a couple times a season and saddle valves fail (by leaking and/or not shutting off) after a half dozen or so cycles. Buy a decent brass quarter-turn valve with a compression fitting sized for whatever supply tube you're using.