A common question from beginners is how long the stitches should actually be in sashiko. Many people try to make each stitch exactly the same length, but in practice that is not always the most important thing.
Traditionally, the stitches should be slightly longer on the top of the fabric than the spaces between them. This gives a clear rhythm to the pattern.
At the same time, it is often more important that the whole works than that each individual stitch is perfect. When lines meet at intersections, you sometimes need to adjust the stitch length a little so that the pattern continues in a neat way.
It may therefore be better to think of the stitches as a steady rhythm rather than exact measurements. With a little practice, the hand will usually find a natural pace and the stitches will become more even on their own.
Sashiko is a craft where repetition is part of the process, and it is often precisely by sewing many stitches that the technique becomes easier.