How to set a default font type and font size in MS Word?

It would have been good if you had also provided the Microsoft Office version you are using because the steps to change the default font size and type vary a little with each version.
To change the default font and size you have to change the default settings of the template. Here I will be explaining the steps by using the Normal template but for all templates it will remain the same.

For Office 2010:

1. Open a new document if you don't have one already open. Select Normal template and click on Create.
2. Go to Font and select Font layout.
3. Select the font size and type you want to apply.
4. Click Set as Default
5. Select the option "All documents based on the Normal.dotm template?" and then click on OK.

For Office 2007:
If you already don't have a document open, select the New option in the Microsoft Office Button, select Blank Document and click on Create.

1. Click on Font option and select the font size and type you want.
2. Click on Default and then Yes to change the default font. Like for Office 2010, the change will be applied at template level, in this case the Normal template.
3. Click OK
4. Click on the Microsoft Office Button and select Word Options.
5. Unhide the hidden commands in the Customize tab by selecting the Commands Not in the Ribbon option and and click on Save All.
6. Click Add > OK > Save All in the customized Quick Access Toolbar.

This should change you default font settings. If you are using some other version of Office then refer the Microsoft Office page for more info .

I use MS Office 2013 (and 2016 tech preview version). So the information mentioned in this reply applies to the specific version.

1. Open MS Word 2013
2. Select Home tab.
3. Choose Styles block from the toolbar.
4. Select the default style from the blocks.
5. Right click on that style and choose "Modify" from the context menu option.
6. You'll find "Modify Style" dialog box.
7. Choose the font of your choice i.e. ' Arial'.
8. Save the settings by clicking on "Ok' Button.

That's all. You can now find that the default font is set to "Arial" whenever new document is loaded.