According with the manufacturer this tray fits the HP Color LaserJet Pro MFP M479fdw and it's available for a great price.
#2 can you choose what tray you want to print from?
Yes, you can. Go to print, not quick print. It will come up in the print box.
#3 What sized paper does this tray hold? Will it hold legal sized paper?
Yes it will, you need to adjust the tray out to accommodate the legal, it does not close all the way but the printer will ask you what size paper is in tray #3, press modify and chose legal... do this after you have selected print on your computer, and selected the options (landscape - legal) once you set up the tray for legal just hit print.