Thats correct, no size restrictions at Don Valley.
If you yourself are not familiar with downtown Toronto, there are several areas that I might stick to rather than heading straight into the heart of it all... plenty to do in the outskirts near the parks, universitiy and shopping districts. The level of construction that is going on there now because of the nice weather seems like it'd be unfair to a dog visiting for the first time - I hated it as a person... this coming from someone who frequents downtown San Francisco. The area directly surrounding The Eaton Center is my main reference. I'd also aim to go on a weekend morning.... I found that to be a lovely time.
Jarvis + Adelaide / St Lawrence Market / waterfront area...
Allan Gardens Botanical Gardens is dog friendly.
Yorkville / Toronto University / Queens park area...
^places that I'd recommend but is in no way all inclusive. It was my first time spending an extended amount of time in Toronto (2 weeks).
You will encounter lots of dogs as well - on leash and well behaved but a lot more than I was expecting - it was nice to see such an active dog loving town. I also imagine it may be a law there (?) but I saw several muzzled dogs too... mainly in train stations, but certainly not the same stigma muzzles carry here in the states. French Bulldogs and Standard Poodles in droves! Beware, lol