The baking bag issue is the important part of your question, so be sure to read to the end before leaving Ochef (if you must leave....)

In general, the size of a roaster in quarts is equal to the size of the turkey in pounds that it can accommodate. That is, a 12-quart roaster can handle a 12-pound turkey. Some roasters we've seen have a little extra room (an 18-quart roaster with room for a 22-pound bird, for example), but that is not true of all of them. If you don't know the capacity of your roaster oven, you might want to measure the interior before you go turkey shopping.

Assuming your roaster is of a more or less standard shape, you should be able to fit an 8-pound bird inside - which is about as small a turkey as you can buy.

Now, on the roasting bag thing, you should not use an oven bag in a roaster oven. Oven bags should never come in contact with the sides or lids of any heating appliance, as it is quite likely to melt the bag. You really have to be sure that the cavity of whatever oven you use is large enough to accommodate the roasting bag when fully inflated without it brushing the sides or top.