I don't know. Spearmint is a much milder mint. I used like 1/4 oz in a Peach Spearmint and that looks to be truning out great. I like the idea of peppermint better.
If you are using freash, I would put it in a hop bag in the secondary. Before you put it in be sure to bruse it up a bit, This will give access to the good oils quicker. I would probably go only with 7-10 leaves at most for a 5-6 gal batch). You would be supprised on how potent fresh is. I also did a Raspberry Thyme and I used only 2-3 sprigs of thyme, I sucked the very small leaves and bruised them, put in secondary for 2 weeks and the 6 gal batch came out with a great thyme smell and a light taste. As I understand that Mint is so much more potent than Thyme.
Making a Tea is a little more work but that might work out too, Keep in mind if you do, the tea will be diluted through the whole volume so it will need to be some potent tea as well.
Hope it works out.
Matrix (Simple, yet Complex)