On Home tab, expand Styles, select one of the items in the list as a starting point (say List Paragraph) and click on New Style (bottom left).
Change Name to something meaningful (say Bullet 12p) and under Formatting enter or select 12 immediately to the right of the font name, OK.
Select Bullet 12p instead of List Paragraph, click on New Style and change Name (say to Bullet 10p). Under formatting enter or select 10 immediately to the right of the font name, OK.
In the Styles part of the Home tab you should now be able to see Bullet 10p and Bullet 12p. If the latter is clicked after selecting the first three lines of text in the example below and the former after selecting the last three lines as below, the results should be of the following nature:
But numerous other elements are controllable, particularly via the Format button on the Create New Style from Formatting window, so "YMMV"!