I think this is a small bug.
Basically SM Player will remember window size and postion in betwen a batch of separate plays and window logons and then after a while the window size and positon will revert back to a box window size of about 400x400 (I dont know precise size)
please rvm anyone check this out if you use the feature like I do which is "remember window size"
might it be that I need to enable 'manual' in the port setting of 'single instance'
[adding a bug report, as suggested, so this doesn't get forgotten.]