Here is another sunken bowl design. Goodman Stadium is at LeHigh University in Pennsylvania and seats 16,000
http://en.wikipedia.org/wiki/Goodman_Stadium . If the field was built to Canadian football dimensions and the lower bowl complete then it looks like it would seat about 30,000. Then for large events more temporary seats could be added above the lower bowl (at street level) in the end zones.
If the upper tier was built so that it partly overhung the lower bowl then it would be a very compact design with a small footprint. Also, it could be built in stages; stage 1 could be 10,000 seats in the lower bowl, stage 2 - complete the lower bowl to give 20,000 seats and then stage 3 could be the partial upper tier for 30,000 permanent seats. If the upper tier was completely surrounding a Canadian field then it would likely be at least 40,000 seats (or more)