The only way I know you can do this is to Build your tables in a Pivot table. The basic table will always allow the user to change the column size its just the way Qlik has been made. this does have its benefits as it allows the user to expand and move the data to make it easier to read.
If you really need the Columns to be fixed then best to use a Pivot Table
Qliksense has a responsive design and this helps in giving a very intuitive look. This is the reason why the columns resize and adjust to the screen resolution. If you still would like to fix the column width, you will have to go with an extension object.