Are you trying to change the text size for new dimensions, or change existing dimension texts?
If you are trying to change existing dimension texts then check to make sure that an 'override' has not been applied to the text size of the existing dimensions.
(If an override has been applied to a dimension style then modifying that style later will not change the override).
More about dimension overrides- http://www.ellenfinkelstein.com/acad...e-a-dimension/
As well as an override on the dimension style, individual dimension elements may have been overridden in the 'properties' dialogue box or by use of the dimedit command.