The area under the 17x/19x seats is not divided as it is on Boeing and Airbus seats. As such, a fairly big suitcase can fit under there, if you're willing to give up a place the put your feet.
Sideways! You'll be giving up your neighbor's foot room too, which is not cool unless you are traveling with a companion.
As SP you'll be in zone 2. Board asap and you should be able to fit a standard roll aboard lengthwise into the overhead.