It depends on the exporter. If you're working in metric units then 1 metre should be equivalent to 1 unit in the game. I had a list somewhere of how big certain objects were but I don't remember them right now.
Metric units are more accurate in terms of exactly how many units you want each polygon's dimensions to be. However, I've noticed that when using imperial units (feet & inches), using the Google guy/girl as a reference is actually perfectly accurate and Google's default obj exporter works totally fine with normal exports. Under this system, Adult Link is just a few inches shorter than the human reference, while Young Link is about waist-high.
To answer your original question, select however much of the model you want to scale, press S, click on one of the highlight corners and type in a scale factor. Simple enough.