Hiding rows has more to do with filtering the underlying dataset to not display the data.
What are you trying to change the size of - the cells or the grid itself? If it's the grid - use the Height and Width properties. If it's the cells, you set the width with the Columns property:
DBGrid1.Columns[0].Width := 50;
I haven't worked with the row height in a regular dbgrid, though (I use TopGrid.)
-Dell