Do you, by chance, have the Find/Replace widget open when you close Sigil? If so, try closing the Find/Replace widget before you close Sigil and see if the window size/position is retained the next time you open.
This was an issue for me with 0.9.12, though 0.9.14 fixed it for me and I haven't had this problem since. However, it seems to still affect some Macs.
More in this thread:
https://www.mobileread.com/forums/sh...17#post3828317