Wayne,
I have made some very satisfying 5x7 in. Pt/Pd images from 6x9 negatives. I think 5x7 may not be too large for 35mm negatives. As for the speed of the original negatives, this probably won't matter, unless you intentionally developed for grain. Since I have a 4x5 enlarger, my interpositives were 4x5. Since you have a 6x7 you have two choices: make the interpositives 35mm and enlarged negs onto 5x7 film; or enlarged interpositives onto 120 film or cut 2-1/4 x 3-1/4 sheet film, and then enlarge the negatives by projection onto 5x7. The latter would require two different sized films and two enlargment process which might degrade the image, and you would have to crop the image twice.
On the other hand, you have a larger interpositive, which will be easier to work with and manipulate.
I would try 5x7 first, as this is a very nice alt. image size; the format is closer to the 35mm aspect ratio, and you won't go broke (as quickly) finding out that larger formats don't work.