It does work, but it is a little confusing.
Open a midi part in the Key or List editor. Make sure that the ‘Length Quantize’ option is visible in the Midi toolbar at the top of the window. (Right click on the toolbar to add the option if it is not visible)
Select the notes you want to set to a fixed length.
Click on the ‘Length Quantize’ option and select the note value you want to set them to.
THEN - Click on the MIDI tab at the very top of the screen , select ‘Functions’ and then ‘Fixed Lengths’.
The notes will then all change to the selected length.
I initially thought that after selecting a note length I needed to click on the ‘L’ to the left of the ‘Length Quantize’ option but this does nothing - it’s the same in N7.
To my mind, it would be more intuitive if the entire process was done within the selected Midi Editor screen.