Click the table button, and select your cell and column size

Insert

Click “table properties”

Set the table width to 100% (the percent is the important part)

Now set each top cell width to the proportionate percentage, so that the total width of all cells equal 100% (EXAMPLE: four columns, 25% width per each top cell. Three columns, 33% width per each top cell. Two columns, 50% width per each top cell. Don’t forget the percentage sign!)