Lets you select objects by dragging the mouse over them.
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Main Toolbar > (Paint Selection Region), on the Selection Region flyout
To change the brush size, right-click the Paint Selection Region button, and then, on the Preference Settings dialog 
General tab Scene Selection group, change the Paint Selection Brush Size value.
If you hold down Ctrl while specifying a region, the affected objects are added to the current selection. Conversely, if you hold down Alt while specifying a region, the affected objects are removed from the current selection.
Tip: You can also create custom tools for changing the brush size; choose Customize menu Customize User Interface and set keyboard shortcuts or other user interface items for the actions Paint Selection Size Up and Paint Selection Size Down.
Customize User Interface and set keyboard shortcuts or other user interface items for the actions Paint Selection Size Up and Paint Selection Size Down. Note: Paint Selection Region respects the Window/Crossing toggle. If the toggle is set to Window and the brush is smaller than an object or sub-object to be selected, you won't be able to select the item. To resolve this, enlarge the brush or choose Crossing.
Procedures
To select by painting a region:
- Choose (Paint Selection Region) from the flyout.
- Drag over the objects to select, then release the mouse button. As you drag, a circle showing the brush radius appears attached to the mouse. Note: To cancel the selection, right-click before you release the mouse.
- To change the brush size, right-click the Paint Selection Region button, and then, on the Preference Settings dialog General tab Scene Selection group, change the Paint Selection Brush Size value.
You can also set keyboard shortcuts for changing the brush size. To do so, use the Paint Selection Size Up and Paint Selection Size Down action items. See Keyboard Panel.