You can use `Quantity` to specify the initial value and domain of a control:

    Manipulate[x, {{x, Quantity[1, "Meters"], "x ="},
                   Quantity[Range[0, 1, .1], "Meters"],
                   ControlType -> Manipulator , 
                   Appearance -> "Labeled"}]
![enter image description here][1]

Few more alternatives:

    Manipulate[Quantity[x, "Meters"], 
     Row[{Control[{{x, 1, "x="}, 0, 1, .1}], Spacer[5], Dynamic@Quantity[x, "Meters"]}]]

    Manipulate[Quantity[x, "Meters"],
     Labeled[Control[{{x, 1, "x="}, 0, 1, .1}], Dynamic@Quantity[x, "Meters"], Right]]



  [1]: https://i.sstatic.net/CVlZC.png