From: https://support.google.com/docs/answer/58183?hl=en

You can change the height of a row or width of a column in a spreadsheet.

  1. Open a spreadsheet.
  2. Click or touch the edge of the header for the row or column.
  3. Drag the edge of the header to the size you want.

Resize multiple rows or columns

On a computer, you can also resize more than one row or column at a time.

  1. Open a spreadsheet.
  2. Click on the header for the first row or column and drag your cursor to expand your selection.
  3. Right-click on the selection and choose Resize columns (or Resize rows).
  4. Change the size in the box that appears.
  5. Click OK.

NOTE: There is a known issue where the row height is automatically changed in Google Sheets after a character has been deleted. This article addresses that issue: http://ask.metafilter.com/281638/Google-Sheets-keeps-doubling-my-row-height-when-I-move-to-another-cell