How powerful is Homura's shield? In the anime, Homura is only seen using her shield to reset or rewind time until the first time she met Madoka.

Can't she rewind time for a minute, or an hour? If she could do it, I think it would be easier for her to fight against Walpurgisnacht.

And how far is its area of effect? When she was chasing a car, she couldn't stop time, but only a few parts were stopped and she still had to run to catch up to it (if she could stop time, she wouldn't need to hurry).
According to Gen Urobuchi from his interview in Otona Anime Vol. 20:


  The weapon that looks like Homura's shield is actually a sand timer. When the flow of the sand is blocked, time is stopped. And when there is no sand on the upper part of the sand timer and then the timer is reverted, one month's worth of time is turned back. But before that stage is reached, only time stopping is possible. This means the special power of Homura is the ability to manipulate one month's time's worth of sand in the sand timer. As for the time passed and then turned back, since it was not the focus in this work I did not think it through very thoroughly. I think you can say that the world split up and became parallel worlds there.


To sum it up:


Homura cannot rewind time until one month of time has passed after March 16th, the start of her time loop.
Before then, Homura can only stop time and the sand in her sand timer from falling.
Homura cannot rewind short periods of time or go forward in time at all.


Besides time, her shield serves the purpose as a probably infinitely spacious storage for all of her artillery. It should also be noted that her shield is quite fragile and clearly not a defensive tool. It can be broken as seen in Rebellion Story when Sayaka stabs the gear mechanism and renders it unusable. (Then again, this is debatable, as the universe Homura created in her witch world bended many laws of magic from the normal universe.)

To explain her constant time-stopping when she was chasing the truck in Ep. 6, one could speculate that she was trying to conserve her magic by periodically stopping and starting time. Perhaps the truck was moving too quickly for the small (fixed?) period of time Homura had to move after stopping time. Note that the average running speed of a human is 8 mph and the truck was probably moving at about 60 mph, just at the speed limit for trucks on highways in Japan.