longer the antenna the better(4 feet or more) and buy an swr meter to see if its working well so you can tune it.
Simple answer- No. (to the first part)
The shorter the antenna, the higher radiation pattern you get (Most of my mobile experience is with 2 meter, and CB is 11 meter)....so following that logic, if you are in a hilly area, a slightly shorter antenna will be better. I get better performance out of my 1/4 wave than my 5/8 wave in the hilly parts of NW PA. In the rolling hills in SW NY, my 5/8 wave does better.
Another thing... The antenna system is by far more important that the radio. You are only allowed 4 watts, so you need a good antenna to radiate that out!
It is more important to spend the $$ on a good antenna system.
Do get the SWR meter, it will help you get things tuned up and help you determine if you are having problems. I have a MFJ meter, I think I spent $29.99 on it at AES Cleveland, although mine is meant for high power Ham Radio use.
Have fun with it...I started with CB when I was 9 or 10 years old, and now I am slowly progressing though the Ham radio tests and levels. (Currently a Technician, will be taking my General test in November sometime)
I am actually going tomorrow to buy my first "advanced" rig...a Yeasu FT-857D. Really looking foreward to it, I kinda feel the same way as I did before I bought my Cobra CB when I was 12.
Radio is fun.