Hi Mokia,
In Magento 1.9.x, there are pre-configured attributes like Weight, but Length, Width, and Height are not pre-configured.
In regards to weight, you can manage the master settings of this attribute within your Magento Admin under Catalog --> Attributes --> Manage Attributes. Find the Weight attribute in the list, and then click on it to see the editable settings.
You can change the weight of a product by going to Catalog --> Manage Products, and editing the value in the Weight field. Normally, the Weight field will be listed in the first tab (the General tab) when editing a product.
If you need to change how Magento handles weights (ex. LBS vs KGS), most shipping methods will allow you to select the appropriate unit of measurement at System --> Configuration --> Sales --> Shipping Methods
Going back to Height and Width, if these already exist in your store, you can add these at Catalog --> Attributes --> Manage Attributes. If they don't exist yet, you'll see a button called <Add New Attribute>. You can create Text Field attributes for these. Once these fields are created. You'll need to proceed to Catalog --> Attributes --> Manage Attribute Sets, select the attribute set(s) you want these fields to apply to (most likely "Default"), and apply this. This involves dragging them from the Unassigned Attributes list to the list of Groups. These groups are the tabs that you see when you're creating or editing an individual product. Without assigning the attributes, the fields will exist in the database, but you won't have a way to use the attributes when creating or editing products.
Lastly, if you're trying to use size dimensions to help calculate shipping, such as if you sell oversized items, you may need an extension for Dimensional/Volumetric shipping.
Best of luck!
