
Will she have free access to the rest of the house from the outhouse? Because if she's used to being with you, and is then shut away somewhere else, she is highly likely to neglect her babies. If she can come and go freely it'll be okay.
For a whelping box you ideally want a square wooden box, at least the same length as her from nosetip to tail. It should be fitted with a rail on the inside, about 5 inches off the floor and 2 or 3 inches in from the sides, so she can't squash the pups against the wall when she lies down. Some bitches are very clumsy with their first litter.
Get her used to sleeping in it as soon as you can, so that when she agrees that it's a suitable place to whelp. Otherwise she'll choose her own place, usually somewhere
really inconvenient, and will get quite hysterical at being thwarted!
:)