Why is my dimensions text always too small?

Or you go to the Standard tools bar, enter Dim edit, Modify the current and increase the text height. simple.

ok. then first of all select the dimension  that you have to enlarge.type the code 'sc' and press enter.select the base point .then enlage it as your wish. type2 for doubling and3 for tripiling  as your wish

see her once the scale........

As mentioned above you need to edit the dimension style. But it's an trial and error process. You might increase it and find it too large only to edit it again. 

It's a good practice to always draw in1:1 scale on model space and use layouts to create viewports showing the particular drawing in a certain scale-that best fits in the target paper space.

Say you have made a drawing, and its depicted in a1:50 scale . A good rule of thumb is to have a dimension style created especially for that scale. Let your arrow and dimension text size be ALWAYS2. And then in Fit...adjust the scale to50. Likewise for1:20 scale, all things remain same only adjust the Fit to20. It's a very handy thing. You shall now see despite having two different drawings on one paper in different scales, the dimensions always appear same size. Size2 is considered ideal for dimensions and text.

If you need a detailed explaination of the above with screenshots..let me know.

have you checked if you have the correct scale factor ...