I use 1.8 on my Janome 6600. It's short enough that the stitches won't come undone
with handling but big enough for the seam ripper when needed. I use a bigger stitch
for general sewing.
Opened seams after washing does not necessarily mean too long of a stitch. It could
be different things such as too narrow of a seam, fabric quality, how it was quilted
or how it was washed or a combination of the above.
There are no rules. Go with what makes you comfortable.