Go for the biggest one you can fit on your counter. The tiny ones are useless since they can barely fit anything, and what does fit in them is so close to the heating elements that it will burn easily. I've turned toast into charcoal twice with cheap small toaster ovens, which also smoked up my place really bad and permanently stained the exterior of the oven to the point where I had to junk them.
I had a larger Oster one as well, and loved it. The only reason I ended up getting rid of it, was because I started cooking too many meat dishes in it uncovered, and the walls inside became a mess with grease. Unlike traditional ovens, mine didn't have that enameled coating, just plain exposed metal (aluminum I think), so everything stuck to it, and oven cleaner wouldn't work. If anything the oven cleaner made it worse, because it discolored all the metal.
They are fantastic for baking stuff particularly when you are in a hurry and don't want to heat your place up with a large oven. They heat up a lot faster because of their compact size. Also, if you are trying to bake two different things that need to cook at very different temperatures, they come in handy for that too.
I would caution though that they do often cook unevenly though since the stuff at the edge of the pan is so close to the heat sources depending on how the unit is designed. So you may need to rotate stuff during the cooking process for better results.
