Do you want to make a square image out of a rectangular image?
An example could help.
If your image is [128, 256]
You can resize it by:
[originalX orignalY] = size(yourImage);
You can match your dimensions by:
newImage = imresize(yourImage, [256, 256], 'Method', 'bicubic');
You can check other methods with the auto-completion... I remember there're cubic, linear, nearest... I think... not sure.
Let us know if that's what you want... If it is not, provide your purpose with some examples... Would be even better if you show what you have tried so far.