I run into this problem a few times. When editing a dimension text, say to add plus/minus to the dim text, I typically use the standard text editor (DDEdit). Occassionally, the in-place text editor box will appear REALLY small, like the size of a pin head, which makes it impossible to read. Oddly, a dimension right next to it done in a previous session appears at a "normal" (readbable) size. 
Reasons?? Is there a configuration somewhere I'm not aware of. Is the annotatioin scales playing havoc with me? Please advise, anyone.