Keeping Text Size the Same Throughout Entire Notebook File

There are a variety of ways to do this. One can use Stylesheets as noted by acl. Perhaps the most direct way is this:

For one Notebook:

SetOptions[EvaluationNotebook[], FontSize -> 16]

For all Notebooks:

SetOptions[$FrontEnd, FontSize -> 16]

You can also set FontSize for different Box types, such as GraphicsBox:

SetOptions[$FrontEnd, GraphicsBoxOptions -> {BaseStyle -> {FontSize -> 15}}]

If you are more comfortable with a GUI, all of these options are available through the Options Inspector in the Format menu.

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If you decide to go the advanced route and use style sheets here is a guide to get you started:

David Park's StyleSheet creation notes (.zip file)

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Depending on your goals, this question may also be of interest:

How to set default magnification for all windows

Yes: you may use stylesheets for this. See also this.