1. "Investment firms" don't build, developers do. Some investment firms may be involved on the financing side but not as development partners. If you look in the Condo Guide, published regularly and available in yellow boxes throughout the city, you will get a good idea who the big players are. Check the index page near the back, particularly, for a quick summary.
2. Depends on the unit mix. Few developments have 3-bedroom units. Typically, 1-bedroom: 500 - 600 sq. ft., 1 bed + den: 600 - 700 sq. ft., 2-bedroom: 700 to 900 sq. ft., 2-bedroom plus den: 800 - 950 sq. ft. Units in "luxury" buildings typically are a little larger across the board. A very few at the high end can run much larger than these ranges.
3. Depends almost entirely on the location, which in turn affects the permitted density. Values are driven by permitted densities, or at least the density that it is anticipated could be achieved upon rezoning. The best sites (close to downtown, west of University Ave.) are now running roughly $800 per square foot. One recent sale on John Street at Mercer Street, directly behind Metro Hall, was reported at $940 per square foot, and another one on Widmer Street south of Adelaide was at just over $1,000, both of which are very high. It drops sharply as you move away from downtown. King Street west of Bathurst is roughly $400 to $500 per square foot. Midtown (Bloor St.) is $500 to $700, unless you are near Yorkville.
Sites in the suburbs are much lower, unless you are within walking distance of a subway station.
4. Good question. Good luck getting a reliable answer to that. It's highly proprietary (ie., confidential) information.