If the stitches were okay when you first started but got smaller as you went along it is most likely due to drag. Keep the fabric up on the table or over your shoulder so it allows the machine to feed it through without the drag. I also agree with sewing in one direction only to prevent puckering. Trying on a sample will let you know if the machine is stitching correctly but it will not help to determine if drag is the problem. Keep trying until you find the solution but maybe on a larger piece of fabric you don't mind losing?