The common advice is that with shaped skis, you should get shorter lengths than you would with traditional, non-parabolic skis. (I'm not sure if Volkl P40's are shaped or not, but if you got them two years ago, I'm guessing they probably are.)
The proper length also depends on your skiing ability level and weight. A good ski shop should be able to recommend the proper ski size - if you get the feeling that the person helping you doesn't quite know what they're talking about, ask to speak to someone else. I know both Ski Chalet and Ski Center have some pros that will patiently work with you to find the right size.
Without any additional information, my best guess is that you wouldn't want anything longer than 178cm, and 178 might be on the high end of what you want.
What is making you have second thoughts about your ski length? How often have you skied in them?
If you get shaped skis that are too long, you will have difficulty controlling them on the slopes - and that's a function of your weight and height more than skiing ability. It can drain the fun out of skiing. (My sister made that mistake; she bought skis that were too long because they were deeply discounted, and that's the only size they had - and she had a miserable time skiing for the next two years until she finally bought a brand new pair.)
Of course, boots and bindings can also affect the quality of the skiing, so if you're having problems, that's something to consider too.
So.. It's possible that 178 might be too long for you. I'd recommend stopping by a ski shop to get their advice. (If it is too long, they might be able to buy your old skis and give you some type of discount on new ones.)
I'd recommend a shorter ski than 178 for you, but your best bet may be to attend a demo day and try out lots and lots of skis. My wife rented all sorts of demo skis before finally settling on a pair of Head Cyberlights.
Another suggestion might be to take a lesson and see what the instructor recommends for your style of skiing.
BTW, your height matters, but your weight is a very important factor too.
My experience is that 188 Platinums ski more like a 184 or a 180, but that p40 F1's ski at their true length or even slightly longer (because of stiffness) compared to other modern shaped skis.
Tom / PM
PS (in edit) - Basically, I'm agreeing with djop with respect to one p40 model, but with Otto with respect to everything else in the p40 line (and his other advice).
[This message has been edited by PhysicsMan (edited 09-12-2002).]
PS (edit #2) - Whoops - for some reason this showed up in my browser as a new post and I didn't notice the date of the previous post until just now. Doh ... oh, well, might as well let it stand.
[This message has been edited by PhysicsMan (edited 09-13-2002).]