The best thing to start with is to go to some of the AWT/Swing tutorials and play around with the layout managers until you get a better idea of what is going on. This is particularly true with the GridBagLayout. If you have access to an
IDE that has a GUI layout tool (such as JBuilder), you can easily throw together a sample UI and start changing the parameters to see what happens.
Each component (java.awt.Component) has several methods that relate to size:
setSize(...);
setMaximunSize(...);
setMininumSize(...);
setPreferredSize(...);
Each LayoutManager uses some subset of those values and overrides other values when placing and sizing components. So the placement and size of a component is a combination of how the component is defined and what layout manager is used.
Generally the "GridBagLayout" will size a cell's height on the highest component in that row and the widest component in that column. Each cell defines what component is there [and you can stick multiple components in a cell, but that's bad technique], the insets, margins, offets, anchor location and x/y-scaling factor. And components can span multiple cells in either the X- or Y-direction, or both.
On a simple level, the GridBagLayout will query each of the components in a column how big they want to be (getPreferredSize()), then adjusts the answer based on insets, margins, offets, etc. It then takes the largest width and that's how wide that column will be. Each row can be a different height, and each column can be a different width.
But notice that you are NOT specifying the height and width when you set up the GridBagConstraints or GridBagLayout. That comes from the components themselves.
So in your last example, the text area will probably be wider than the radio button, so the width of the first column will be enough to accomodate the text area, and the radio button will have lots of extra room.
Again, look for some tutorials and play around with each of the Layout Managers, but particularly on the GridBagLayout if that's what you want to learn and use.