I am running out of ideas, but it worked here until I have decided to play with preference then it decided not to work anymore... no time to investigate, but then I suspect that I am using the same personal directory for 1.4 and 1.5.
Not all session stuff is in the preference dialog yet. There should be 'save windows position' (a checkbox) and windows width and height (two edit boxes) that allow you to set fixed windows size. I do not have time to check this right now, but you can at least have a look at ~/.lyx/session and see if you have WindowHeight and WindowWidth values saved and changed when you close lyx. Bo
