I'm not aware of any direct way of using a gradient for scale. I'm not sure Max can even read a gradient translating to time. It's not that difficult to use the scale though.
If you sync by abosulte time, it will animate every particle on the frames that you've keyframed. ie: If you have a key on frame 20 and another on frame 40, all particles will animate scale in unison from 20-40.
If you sync by particle age or event age, frame 0 becomes the starting point for each particle's scale animation. ie: If you have the same 20-40 keyframe animation and set to sync by event age, each particle's scale will start to animate 21 frames after they enter the event.
3ds Max 2009 SP1, 2010 SP1
Maya 2012
Windows 7 Professional 64-bit
Dual Intel Xeon E5520, 6 GB Ram
NVIDIA GeForce GTX 460 OC