When on, all the windows are automatically made the same size after splitting or closing a window. This also happens the moment the option is switched on. When off, splitting a window will reduce the size of the current window and leave the other windows the same. When closing a window the extra lines are given to the window next to it (depending on 'splitbelow' and 'splitright'). When mixing vertically and horizontally split windows, a minimal size is computed and some windows may be larger if there is room. The 'eadirection' option tells in which direction the size is affected. Changing the height and width of a window can be avoided by setting 'winfixheight' and 'winfixwidth', respectively. If a window size is specified when creating a new window sizes are currently not equalized (it's complicated, but may be implemented in the future).