The usefulness of the following depends upon the simplicity of the ratio of the scaling you desire (or, how good you are at maths! 
)…
Let’s take the simple example that you wish to double the tempo of your MIDI Parts and their contained events (and, in this example, that the original tempo is 120 BPM)…
Set all the tracks for which you want to do the scaling, from Musical Timebase to Linear Timebase (“quarter-note” icon in the track’s Inspector changed to “clock” icon)
Change the tempo from 120 BPM to 60 BPM. (those tracks still “sound” the same, although now against a tempo of 60 BPM).
Set the tracks back to Musical Tilmebase.
Change the tempo back to 120 BPM.
Done .
Alternatively, if you’d rather use the “Sizing applies Time Stretch” tool, simply glue all the parts together, use the tool, then split the parts again .
Or, as Steve says, the Logical Editor/Project Logical Editor .