How do you insert a text box that will not change sizes, but will change the text size to fit that box?
Can you elaborate with an example of what it is you are trying to accomplish as an end result? On the surface, with your current description, the answer may well be No.
I remember from school where we set the limit of our text box (such as in a title block), then as we typed in that box the text would decrease in size as necessary to make all text fit into that box.
I believe (could be wrong, someone will correct me soon) you are describing TEXTFIT Express Tools command, not an option in LT: did school teach LT, of full AutoCAD?
The FIT justification for single-line text adjusts the width factor (but not the height) to accommodate different numbers of characters. I'm not sure if that would meet your needs, but it's an option.