Platforms should be the same length (152 metres) but who knows what equipment they will use, or the actual train lengths.
Ridership is forecast taking population growth into account.
Too early to tell really ... if one is optimistic and it opens in the early 2030s, then there's two choices. One is that they'll include the line in the order to replace the T1 equipment. The other is the T1 replacement equipment will be used on Line 1, and the 6-car TR equipment will be used on Line 2 and the new Downtown line.
If it's the new T1 replacement equipment, we don't even know the car lengths. The talk is they want to use the entire 152 metre platform, instead of the current 6-car trains which are only about 138 metres long. But whether that means 7-car trains (about 22-metre long cars), 8-car trains (19-metre long cars), or 9-car trains (17-metre long cars - same length that Montreal uses, or Toronto originally used) is not yet clear. 6-car trains appear to be out though as I don't believe 25-metre long cars will around the curves.
Given they've been talking about this line, in this general configuration for over 50 years now (and in other configurations for 70 years now), who knows when it will actually be built, or what equipment it will use!