Hi folks.
The Tape Measure Tool will scale along all the directions at once and yes, if you dont want to rescale the whole model but only a part of it, it must be used in the context of a group or a component.
However, to rescale only in one direction, the tool to use is either the Move Tool, used with a carefull selection, or the Scale Tool.
When using the Scale Tool, if you use a pure number, the number will be used as a scale factor.
If you follow the number by a unit suffix like mm (for millimeters), for example, the real dimension will be used. Thus, even if you want to use your default unit, you must enter it after a number if you dont want the Scale Tool to consider the inputted number as a scale factor.
And lastly, if you use the Move Tool or the Scale Tool, the object do not need to be in a group or a component.
Just ideas.