I make my stitches for my SID about 5 and use invisible thread on the top amd matching thread on the bottom. The invisible thread does not show so if you are not perfect with your SID it does not show. I often do not use a walking foot but rather use my regular foot and raise the presser foot tension and keep the feeddogs up. Also your quilt may be dragging causing the stitches to be crooked and actually become smaller. If you can try to get a larger sewing area that will support your quilt. Here is a link to the sewing machine table that my DH made me and it was cheap and works great.
http://www.youtube.com/watch?v=g14govA4pIM I ususally keep this table up and on my machine all the time and it gives me a great work surface. One key to sucess is to have your machine even with any work space that you have so the quilt is not drag.Even if you have an extension table 0n your machine the quilt will still drag causing problems. You can either drop your machine to table level or raise table level to the machine height. as this linek shows. Good luck!!!!! You can do it!!!!!