The Position page in the Cell Format dialog lets you alter the size of the selected cell(s). Note that changing the height of a single cell will change the height for all cells in that row, similarly changing the width will affect the entire column.

You can also select the row(s) or column(s) to be changed then select Resize Row... or Resize Column... from the right mouse button pop up menu or from the Format → Row or Format → Column menu.

If you move the mouse cursor so that its tip is over the line between two of the row numbers at the left of Calligra Sheets's window the cursor will change to show two parallel lines each with a short arrow headed line coming from it. When the cursor is in this state you can hold the left mouse button down and drag the border between the two rows, changing the height of the upper row. A similar technique can be used to change the width of a column.

To set the row height or column width to the minimum needed to display the contents, select the whole row or column, and click with the right mouse button on the row or column label. In the menu which appears, select Adjust Row or Adjust Column. The row or column will resize to the minimum necessary. You can also select a single cell or range of cells, and click Adjust Row & Column from either the right mouse button popup menu or the Format menu.

You can make a number of adjacent rows or columns the same size by selecting them then choosing Format → Row → Equalize Row or Format → Column → Equalize Column.