Don't use bwselect(), use bwareaopen. It does exactly what you asked for. From the help:
BW2 = bwareaopen(BW,P) removes all connected components (objects) that have fewer than P pixels from the binary image BW, producing another binary image, BW2.
Here is what you'd do:
binaryImage = bwareaopen(binaryImage, 251);
You might also be interested in bwareafilt() which extracts the N largest or smallest blobs, or blobs within a specified range of areas. It can also do what you want.
binaryImage = bwareafilt(binaryImage, [251, inf]);