You can cook just about any size cut of beef in a slow cooker because the well-insulated, ceramic crock keeps everything inside moving along at the same rate, like an oven braise but with less variation in temperature and moisture loss. Slow cookers have tight-fitting lids, so the only variable you need to consider is how much liquid to add -- a few tablespoons if cooking beef cubes by themselves, and a few cups if making a stew -- because as moisture evaporates, it collects on the lid and drips back down into the crock. If you add vegetables to the meat, cut them in roughly equal-sized pieces.
Season the beef cubes with your favorite spice mix or simply kosher salt and cracked black pepper and add them to the slow cooker.
Add 3 or 4 cups of stock if you want to make a stew and about 1/2 cup of stock if you want to cook the meat cubes by themselves.
Add vegetables to the slow cooker if desired. Use a basic mirepoix, or chopped onions, carrots and celery to start with, and build from there. No matter what vegetables you choose, cut them into pieces of roughly the same size.
Add 1 to 2 tablespoons of all-purpose flour for every cup of stock if you're making a stew. Although you rarely add raw flour by itself when cooking a sauce or soup on the stove, slow cookers take so long that the flour combines with the fat from the beef early on, and the starch cooks out within an hour or two.
Cover the slow cooker with the lid and set the temperature to low or high, depending on when you want to serve the meat cubes. It takes about three to four hours to cook meat cubes on high, and six to seven hours to cook them on low.
Take the beef cubes from the slow cooker when they finish cooking and serve immediately. There's no need to rest the meat when you cook it in a slow cooker. If you made a stew, taste it when it finishes cooking, and adjust the seasonings as needed before serving.