You are mixing 2 of the variables. in the documentation we can see the following:

BW2 = bwareaopen(BW, P)
BW2 = bwareaopen(BW, P, conn)

Where you have P and conn variables.

conn is the neighbourhood variable. With this you chose with values you understand as "neigbour". See example:

conn=4;

- X -
X 0 X
- X - 

conn=8

X X X
X 0 X
X X X

Higher numbers are for higher dimensions. but the P variable is the variable to choose what is "small". The function bwareaopen will delete white areas smaller than P pixels.

In the example in the documentation you can see:

    BW = imread('text.png');
    BW2 = bwareaopen(BW, 50);
    imshow(BW);

But that "50" is the P variable, not conn. as conn is not defined it will be set as default, in this case 8.

Take a look at the cvBlobsLib, it has functions to do what you want. In fact, the code example on the front page of that link does exactly what you want, I think. Essentially, you can use CBlobResult to perform connected-component labeling on your binary image, and then call Filter to exclude blobs according to your criteria.