when i'm using viewports, my text heights are either too small or too big. please help to resolve this?? THANKS!

Sounds like you need to make your text larger or smaller :)
 
OK....if you will be plotting at only one scale, set your text height so it will be the desired height at that scale.
If you expect to text to show up on different plots/different scales, you could;
-include the text multiple times, on multiple layers, and control layer visibility in the viewports
  ...or..
-give the text different annotative scales, it will dispaly at the MS set height in the viewports that match the text's annotative scales

I would do, different viewport with different text style, means different text heigth unless the viewport scale is same.