Assuming that you have applied language formatting to all of the text, you should be able to do this via Find/Replace.
In Find->Replace, click the "More > > " button if necessary to show the bottom part of the dialog.
Click in the "Find What" box, then click "No Formatting." Then click Format->Language..., and select the language, e.g. Arabic (In the case of Arabic, I think you will need to specify the variant you have used. If you have used more than one, you may need to repeat this process for each).
Then click in the "Replace With". Click "No Formatting" again to clear any formatting. Then click Format->Font... This is where the "catch" is: if you want to specify the font size for the LTR text, you need to specify it in the "Size:" box of the "Latin text" section of the dialog (you can probably ignore the fact that there is also an "Asian text" label there - both types of text share a size. If you want to specify the font size for the RTL text, you need to use the "Size:" box in the "Complex Scripts" section of the dialog.
After that, you should be able to use Replace All to modify the size of all the LTR or RTL text.
If you get that far, it may look as if you could replace the sizes of both types of text all in one by specifying both sizes in "Replace With" and no language in "Find What", but it doesn't work because the relevant buttons remain greyed until you select something in the Find What box.
Best Answer
If all the larger text in your document will be the same font size, and all the smaller text will be the same, select one bit of smaller text, and then, in the Font dialog box, on the Advanced tab, select Raised in the Position field, and then set the By value to a value that puts the text at the height that you want. Then create a character style for the smaller text. If the correctly styled smaller text is selected when you create the new style, the new style will have the properties of that text by default. (Instead of creating a new style, you could repurpose one of Word's existing styles, updating it to match the style of the smaller text). Then apply the style to all relevant text.
Alternatively, depending on the characteristics of the specific font that you're using, and the specific sizes that you're using, just styling the smaller text as a superscript by clicking the Superscript button might get you vertical centered text, or at least close enough to look convincing.