How to fit word to width with ffmpeg drawtext filter?
There's no way to force the drawtext filter to fit the text in a certain width.

What you can do, is get the rendered text dimensions in a first pass and then run drawtext accordingly in 2nd pass, or crop and scale the output using the values from first pass.

For first pass,

ffmpeg -i video -vf drawtext=fontfile=path/to/font:fontsize=FinalSize:text='Lorem Ipsor' \
                   :x=0+0*print(tw):y=0+0*print(th) -vframes 1 -f null -


This will print the text at the top-left. The print statements will output

84.000000
14.000000


The first value is tw - the width, and the 2nd th - the height, in pixels.