Why is my dimensions text always too small?

• For getting dimension visual & in proportion::

Press D Enter ( D + Enter on keyboard)

You will see... Dimension Style Manager

Dimension Style Manager creates new styles, sets the current style, modifies styles, sets overrides on the current style, and compares styles.

Click Modify in Dimension Style Manager

Modify displays the Modify Dimension Styles dialog box, in which you can modify dimension styles. Dialog box options are identical to those in the New Dimension Style dialog box

In modify select FIT & just change overall scale to desired size.

Plus point of changing scale is you will get equal proportion between both text and arrows.

Save the settings.

(here is modified dimension after changing scale…it is visible & in proportion!)

This will solve your Dimension size problem.

it's very general question you need to be more specific since this issue can be occurred because of many reasons , for example :

1. using annotation and not setting up the scale 
2. the dimension text is looking small on the model or layout or both 
3. the file or block unite is not matching with the original fie in case you include it from on any blocks or X-ref ?
4. your file unit by Meter& you drafting using MM 

 best practice is to create you own dimension style from scratch and importing it when needed

•  you need to know what will be your unit is it MM , CM , M ,inch etc
• try to make just a box 1 unit X 1 unit and take linear dimension
• go to dimension style manager   go FIT tab and adjust the overall scale 
• create your own dimension style by adjusting the value of Baseline spacing , Extension lines ,Arrow head ETC 

I am agree with Madam  Elke Woofter.

Are your default settings correct?

What scale are you coping from? If it is the scale adjust the scale

see if this website is helpsful 

https://knowledge.autodesk.com/support/autocad/learn-explore/caas/CloudHelp/cloudhelp/2016/ENU/AutoCAD-Core/files/GUID-38A3E134-3CCD-4EAF-90B8-A4EDA83F36FF-htm.html

To change text height.. you can go into modification & change the height .