Annotate image with filled text by image size

[Eugene]

I have multiple images and now I need to annotate them with the single text line. The problem is that I need to maximize the size of the text to fill source image.
If I choose the pointsize option it gives me constant text size and it does not work good.
Text is variable by characters size and I cannot set constant pointsize.
Everything I try to do via batch file in Windows 8.1 Pro.

Please help me to annotate my images.

How to solve my problem?

[Eugene]

PS
My text is a single line, but I need to fill with it whole image. So it's probably have to wrap into multiple lines on the final image.

[fmw42]

Use label: with a given width and transparent background to create a new image. Then composite that image over your other image.

see
http://www.imagemagick.org/Usage/text/#label