The underdrive pulleys are Bigger, not smaller. Most companies sell either a single crank underdrive pulley or as a pair, crank and alternator. You will need about a 95" serpentine belt if you run an underdrive crank pulley and standard alt. pulley. (this is the setup I have on my truck now). It is a PITA to find a belt to match up to use both underdrive pulleys. Its roughly a 97-98" belt needed to use the stock tensioner. I ran both on my truck initially, I wound up using different diameter tensioner pulleys to make it work.
I try to break it down with simple Yes or No

Yes- you will see considerable voltage drop at idle running both pulleys(about 10 volts at idle. Around 11 at idle with single crank underdrive)
Yes- The Voltage will come back up as you accelerate or raise idle.
Yes you may encounter cooling issues if you live in a hot climate. I live in NJ, my truck in summer heat and A/C on has never run over 200.
Yes- You may feel/see a difference after installing them. I noticed a 200 rpm drop at 70 mph on highway.
Yes- Installing the crank pulley on a 5.2 is as simple as remove bolt,removed stock,install new, tighten and retorque bolt. On a 5.9 you have the harmonic balancer to deal with.

No- This is not a "power adder" mod per say. But increased accessory life and the engine working less(read mpg).
No- If you constantly run alot of lights,winches,high powered stereo. This might now be the greatest mod for you. Especially if you do alot of stop and go driving.

 

^^ Okay to clarify: Underdrive crank pully will be smaller, any other underdrive pully will be larger.

Right??