type x vent

The reason is "port noise" by using vent openings that are too small or too oddly mishaped.

Vent opening size ratio should be 8:1 or less.   For example, 16" x 2" or 12" x 1.5".  That's maximum.  More square is better.

If the website gives you dimensions for a 11 X 1 vent and you want to change the long side to 15", you have to...

Obviously they are not following the 8:1 ratio rule when they give you build plans using a 11 X 1 slot opening.  But, nevertheless, from that you know that they intend that the slot opening area should be 11 square inches in order to correspond to the vent length that they specified.

(As an aside, if you have room for a 15 X 1 slot, you also have room for an 11 X 1 slot)...........

So we look at rule #1 and see that an 8:1 ratio or less should be attained.  11 X 1 doesn't make it and neither does 15 X 1.

To make your port the length that is specified, find a length to width ratio that is acceptable and that also equals 11 square inches.  As long as the opening size in square inches is the same as the specification, the length of that  port remains the same as specified.

I am sorry in advance for everyone taking this the wrong way...(two weeks vacation in hawaii, then I had to come back to work)

You obviously don't visit the real world all that often. People that run huge money making businesses do not do it for the customer. They do it for the huge money. Don't get me wrong, I agree with your views on the subject, but I can't think of a large corporation that the owner has done anything to show me that they care about the money less than the customer. I get stuff in the mail all the time from various companies that say "because you are a valued customer" and then go on to tell me about something that I will be spending money on. Any time money is given out it is usually for tax write offs and lawsuit evasion. Most product development is not for making stuff last longer or sound better, it is for a subtle change, sometimes cosmetic that makes this years model a "must have" so you go out and spend more money to replace a perfectly good unit.  The manual was not written by an engineer, it was problably typed by a malaysian sweat shop worker that does not have a car much less a stereo to put in a car, and probably does not speak english. The only 8:1 ratio that they know is they work 8 hours and get 1 dollar. It very well could be a misprint.

What do I know. I am a sucker for new gum brands and flavors.