From Select Layout dialog you need to pick wanted layout of columns, e.g. 1/2+1/2 will add two columns with same width.
Note that Row element is based on Bootstrap’s 12 column grid, meaning you can have 1,2,3,4,6, and 12 columns in a row. Once you select the number of columns, you can set their width as well.
For example, the following row has two columns where left column is 3/4, and right one is 1/4.