I thought, It would help to you.
Ext JS docs says:
Setting Sizes
The columns are laid out by a Ext.layout.container.HBox layout, so a column can either be given an explicit width value or a flex configuration. If no width is specified the grid will automatically the size the column to 100px. For column groups, the size is calculated by measuring the width of the child columns, so a width option should not be specified in that case.