The whole quilt must be supported around the machine or you will get drag which will effect the stitch length. Unless your machine is set into a table, even catching on the machine edge or the surround will hold up the sandwich. Stitching through more seam allowances in some spots with also effect the stitch size, the more layers to stitch through, the smaller/tighter the stitches.
Another thing to check....on my Bernina the walking foot attaches to the machine BUT there is an arm the must go onto the needle bar where you tighten the needle. If your machine is the same, check that the bar is where it is supposed to be or the walking foot has trouble " walking"