Repro Steps:

1) Create a new Page.
2) Drag in a Slider Element
3) Right-Click on the Slider and Select Template-->Create New

From within the new Slider template:
4) Drag in a 'Track' Element from the Elements panel.
5) Rename the dragged-in Track to 'PART_Track'.
6) Set the 'Thumb' Property of the Track to "Thumb"

From within the Resources panel:
7) Create a ControlTemplate.
8) Set the ControlTemplate to be of TargetType=Thumb
9) Create a Thumb Visual (a red ellipse works for this) within the ControlTemplate. Do not set a Width or Height value for this shape.

From within the Slider Template:
10) Under the "Thumb" Properties of the Track, under the 'Miscellaneous' section, set the Template to be the ControlTemplate created in Step 7).
11) Under the Track Properties; set:
11.1) The Value to be bound to TemplatedParent/Value
11.2) The Minimum to be bound to TemplatedParent/Minimum
11.3) The Maximum to be bound to TemplatedParent/Maximum

From within the highest level of the Slider itself, in its Properties panel:
12) Set the Value to '5', the Minimum to '0', and the Maximum to '10'

Observe how no Slider Thumb is visible within the Slider.

13) Navigate into the Slider Thumb Resource, and set a fixed Height and Width.
14) Return to the Slider Element.

Observe how the Thumb is now visible.

Expected Results: Slider Thumbs inherit the dimensions of the Slider in which they exist, and can either be set to Stretch or an Alignment Mode like any other children elements.