It depends on your pattern. You can shrink the block sizes (example: from 12" to 9"), you can shrink the number of blocks (example: instead of 5 rows x 6 columns of blocks do 4 rows x 5 columns) or you can alter the borders (make the borders not as wide and/or don't use all the borders the pattern calls for). Some patterns lend themselves more readily to different ways of resizing.
Do you have a specific pattern in mind? If so, share it, and we may be able to give better suggestions.
You can also use these methods to make the quilt bigger (increase the block size, increase the number of blocks, increase the border width, and/or increase the number of borders)