Interesting discussion here. Yes, Barclay's Center is another great example of an "urban" stadium project built recently. NIMBYism could be a negative factor towards the Dundas West site, however I would think that the Bloor West BIA (or whatever group happens to cover that area, not sure how far their territory actually extends) would be quite keen on the project, seeing as how it would give a direct boost to the restaurants and bars along Bloor, and also along Roncesvalles. This would also help disperse crowds before and after the game, since people would go for dinner before or for drinks after.
As for stadium size, here's some comparisons of "Neo Classical" stadiums:
Camden Yards (Baltimore): 45,971 (plus standing room)
PNC Park (Pittsburgh): 38,362
Target Field (Minneapolis): 39,021
Nationals Park (Washington): 41,418
Petco Park (San Diego): 42,302
AT&T Park (San Francisco): 41,915
So it looks like the sweet spot for modern MLB parks is between 38,000 and 42,000, with optional areas for standing room for big events like playoff games. If there ends up being a space constraint in left field due to the rail tracks, they could conceivably make a pretty steep "standing room section" above the left field wall, similar to the Green Monster in Boston, only without the seats. Another example of this is the Western Metal Supply Co. building that was incorporated into Petco Park. It features several levels of vertical standing room decks. This would add extra capacity for big games, but would keep the stadium feeling full for regular weeknight games.