how to reduce the size of a matrix ?

I have 7 tiff files each with (11055,18845) number of rows and columns.while reading them as tiff file in matlab I get memory error. How can I reduce their size or overcome this memory error?can I reduce the number of rows and column since most of the values are nan?

[John D'Errico]

Please stop asking the same question over and over again. You have gotten answers on the last few virtually identical questions you asked.