Cooking large sweet potatoes in the oven can take close to an hour, which, if you're in a rush, doesn't lend itself to cooking those large sweet potatoes.
An alternative—which also helps in portion control—is to cut the potato into smaller pieces (slices, wedges or cubes are fine) before roasting. Use a toaster oven set at 350 degrees and you'll have soft pieces of sweet potato in 15 to 20 minutes. Make the temperature a little hotter and you may save yourself a couple more minutes!
I actually prefer Japanese sweet potatoes (in photo) to the more common orange sweet potatoes. The flesh of Japanese sweet potatoes is a yellowy-white, while the skin (eat it!) is a darkish red. It's a little drier than an orange sweet potato and has a slightly nutty flavor.
Many Whole Foods stores sell organic Japanese sweet potatoes (grown in California). I also buy them from Asian farmers at local farmers' markets.