On my Motif XS7 how can create crescendo and decrescendo effects on a track while I am recording? I an not “live” recording. I am doing it by track.

Can I specify a specific measure and how long I need the crescendo/decrescendo to last?

Unless you have a very steady hand (or foot) I’d recommend overdubbing expression because if (or more likely when) you mess up you can undo and do over without messing up the recorded notes.

You’re best to use CC11 (Expression) as opposed to CC7 (Main Volume). Be warned, though, controller messages like this are very data hungry so any crescendo / decrescendo is going to eat up a lot of memory. Depending on what type of part you’re doing this on you might find a foot controller gives you the smoothest transition. Long fades (more than, say, 2 bars) are going to be very hard to create smoothly in real time but, as MIDI data, you can always go into event edit and make the changes more even.