I believe I'm the biggest Mucho Burrito fan out there and I live in San Francisco where a world-class burrito is a commodity. I've gotta say that this is the only place in Toronto that measures up (I lived in Toronto for 5 years).
I almost exclusively order the medium chicken burrito. The medium will satisfy you, the grande size is extremely large and most likely you won't be able to finish it.
They'll walk you through each step in the burrito building process, so I'm going to let you in on my secret. Make sure you ask for extra grilled veggies at the early stages, as you get to the end of the line make sure to ask for extra extra burrito sauce (that's right, you have to say extra twice!). I have no idea what the sauce is, but it's damn good and there's no time to question.
If you follow my two tips, I guarantee this will be the best burrito you've tried in Toronto, hands down.