Go to Format on the menu bar and select Style, choose the style the dimensions are in from the list, then select Modify, the text size can be altered under the Text tab.
Be aware that changing the text size of a dimension style will change all text that uses that style.
I've messed up drawing borders etc by doing this.
I prefer to have dimensions with their own format, based on, for example, ISO standard format. You can then force all dimensions to use this format with Tools>Options>Dimension Style.
This also lets you set up different styles for Balloons, Notes, Radial Dimensions etc.
