If you find out that you always need to print your parts a big bigger, then I would use an import action. It will automatically scale any parts that you import, for example, if you determine that your parts always shrink by 0.5%. This video explains how to set it up:
https://www.youtube.com/watch?v=E8ytvwfCn5g
As far as scaling multiple models at once, you just need to select the model you want to change before scaling them. For example, they do that at 2:59 in this video (
https://www.youtube.com/watch?v=HIhpUfCorSM) and also a similar video about selecting and grouping if you haven't done this before (
https://www.youtube.com/watch?v=ZQFhDZcEPhc)