When you add text to a symbol, it will automatically expand in size to fit the text while you type.
You can apply a more uniform look after you've finished adding text. Select the symbols (How?) you'd like to make uniform and in the Design tab click Make Same. You can make the selected symbols all have the same height, width, or both.
To resize an item, select it and black square handles will appear on it. Click and drag these handles to resize that item.