There are several ways you can change the size of text that appears as alphanumeric information in drawing annotations and dimensions. You can:
On the drawing, right-click the element you want to change, such as a bend callout or a balloon.
From the shortcut menu, choose Properties.
In the Properties dialog box for the element you selected, click one of these tabs:
Do one of the following:
Change font size—Type a different value in the Font size box.
Change overall text size—Type a different value in the Text scale box to increase or decrease the overall size of the annotation text.
Change the width of callout text and symbols—Type a value in the Aspect ratio box to change the text width but not text height.
Click OK to apply the change.
You can use SmartSelect to select all elements on the active sheet that have similar attributes. This is very useful if you want to make changes to similar elements throughout a drawing sheet.
Click the Select tool 
.
On the Select tool command bar, click the SmartSelect command 
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Click one of the text elements you want to change, such as a balloon or callout annotation or a dimension.
The SmartSelect Options dialog box is displayed, with the Element Type check box selected.
Click OK to continue.
The Edit Definition command bar is displayed.
To change the text size of all of the selected elements, in the Text scale box, type a new value and press Tab.
The text size of all of the selected elements is changed.
Tip:
You can use the other options on the Edit Definition command bar to make changes to the elements in the select set. For example, you can select the Properties button to change the color of the elements or text.
This method updates all existing text elements that use the text style, and it is applied to all new elements that you create.
Choose the Home tab→Dimension group→Styles command 
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Tip:
You can find the same Styles command on the View tab and on various annotation Properties dialog boxes.
In the Style dialog box, from the Style Type list, select Text.
From the Styles list, choose the Style you want to modify, such as ANSI or ISO.
Click Modify.
In the Modify Text Box Style dialog box, click the Paragraph tab.
Do one of the following:
Change font size—Type a different value in the Font size box.
Change the width of callout text and symbols—Type a value in the Aspect ratio box to change the text width but not text height.
Click OK to dismiss the Modify Text Box Style dialog box, and then click Apply to update the text style wherever it is used.
Tip:
If you modify the style of Dimensions rather than Text, you can specify a value for the Callout text aspect ratio on the Text page of the Modify Dimension Style dialog box.