rayray, a slow cooker uses about the same power as a lightbulb (not sure what wattage). It makes sense - an oven heats your food and a whole heap of empty space while a slowcooker just heats the food
Caro, I don't usually change the quantities. If it looks like your SC might be really full it might be best to allow a longer cooking time. Mine is big enough for a lamb leg and I find that things cook a tad faster in it than most of the recipes indicate. I'm still not sure if it's because it's big or just a little hotter.