Flutter - how to remove default padding (48 px as per doc) from widgets (IconButton, CheckBox, FlatButton)

wrap your CheckBox inside SizedBox will resize the padding of the check box

  SizedBox(
    height: 24.0,
    width: 24.0,
    child: Checkbox(...),
 )

EDIT

Set the value of materialTapTargetSize to MaterialTapTargetSize.shrinkWrap

Checkbox(
   materialTapTargetSize: MaterialTapTargetSize.shrinkWrap,
    ...
 )

Alternatively, you can achieve this by customising the Checkbox widget.

1. Create a CustomCheckbox using the exact code from flutter/packages/flutter/lib/src/material/checkbox.dart.

2. Add a new field to your CustomCheckbox widget

        final bool useTapTarget;
   

3. Make sure to add the new field to your constructor with it default value set to true.

        this.useTapTarget = true
   

4. Modify the build method in the _CheckboxState method. Add this block of code above the return call.

        Size noTapTargetSize = Size(CustomCheckbox.width, 
        CustomCheckbox.width);
        final BoxConstraints additionalConstraints = 
        BoxConstraints.tight(widget
           .useTapTarget? size : noTapTargetSize);
   

5. Finally, use your CustomCheckbox widget in your code, and set your custom field to false to remove material padding. example

   Container(
           margin: EdgeInsets.only(right: 15),
           child:CustomCheckbox(
               value: _checked,
               onChanged: _onCheckBoxChange,
               useTapTarget: false,
               activeColor: Colors.teal),
         )
   

There are three options:

1.

 // or Checkbox()
Radio( 
    materialTapTargetSize: MaterialTapTargetSize.shrinkWrap,
)

 // or Checkbox()
Radio(
    visualDensity: VisualDensity(horizontal: -4, vertical: -4),
)

SizedBox(
    height: 20, 
    width: 20, 
 // or Checkbox()
    child: Radio()
    )