Writing Text in Cells in Numbers
Long text when Wrap text in cell is turned off does not merge with the next cell, it simply overlays on top of the adjacent cell if the adjacent cell is empty.  Numbers just hides the borders for readability.  If the adjacent cell has content, the text hides behind it.  

There is no way, unfortunately, to expand the width of the cell to fit without expanding the width of the entire column.

Experiment with the text wrapping to get your desired result.

Hope this is useful!