The key to getting a dynamic block to work the way you want is to specify exactly how you want the dynamic properties to behave. The dynamic block in the exercise file includes parameters that enable you to stretch the length of a dynamic block, but there are no restrictions on the length of the block.
In reality, light fixtures are only available in standard sizes. You can edit the properties of the linear constraint parameter so that the block can only take on the sizes that you specify.
Any time you want to make a change to a constraint parameter, you must go into the Block Editor and then change the properties of the parameter.
To open the Block Editor, double-click the block. Once the Block Editor environment is active, you can see the linear constraint parameter that had previously been created.
Select the linear constraint parameter and then right-click and choose Properties from the shortcut menu to open the Properties palette and display the properties of the parameter.
In the Properties palette, one of the value sets allows you to set what is known as a distance type. Change the Dist Type to Increment. Then you can set the distance increment. Set an increment value of 24 units. This will allow you to be able to stretch the light fixture only in 2-foot increments.
Then, set the Dist minimum value to 2′ and the Dist maximum value to 8′.
Now that you have made those changes, click the Test Block button and test the block to make sure it behaves as you need it to. This time, when you select the block and select the grip, you can only stretch the block in 2-foot increments. You cannot make the length anything less than 2 feet, and you cannot make it any longer than 8 feet.
Now that you know that the block behaves the way you want it to, you can close the Test Block window, close the Block Editor, and save your changes. If you select an instance of the block in the drawing, it, too, now has those same dynamic capabilities.