Well these things are always subject to change, but the
draft internal layouts were published by DfT last year.
The specification states a ratio of 35% bay seating (I assume this is table seating) and 65% unidirectional seating. It allows for variable seat pitch, but specifies minimum 'knee room' in standard class of 750mm (29.5") for unidirectional seats and 735mm (29") for bay seats.
For comparison the seat pitch on easyJet is 29" - meaning the 'knee room' is much less than that. I don't know how 'knee room' is officially defined, but
this post from 2010 which uses a definition of "the distance from the rear of the seat (parallel with the seat cushion) to the point on the rear of the seat in front at the height where one's knees would normally be" suggests that the IEP knee room will be on par with a Class 185, Pendolino or EMT HST and more generous than an EC/FGW/XC HST, Class 158 or a MkIV coach.
Edit: the Transport Secretary's statement says the 9-car trains will have 627 seats. In the draft layout (linked above), the 9-car train had a total of 636 seats (526 standard and 10 first) - so I expect the layout to be very similar to the draft.