Hamster babies and You

Ihave two hamsters: Demitri (boy) and Hazel (girl) Demitri is gray and fat with black eyes and Hazel is white, brown, black and skinny with red eyes. I put them together in a a giant box thats only about 10 in. high and they mate every time shes in heat, so all of a sudden she starts acting different, eating more, sleeping in this one particular corner more, but she hasnt been drinking more. so I give her some toilet paper so she can build a nest and she does. I just changed her bedding and her toilet paper and I took all the decor out except for the water bottle and the food dish, but she still hasnt gotten fat! did she just have a growth spurt or, did she just build the nest so she can be comfertable? I dunno. also,how many babies will she have? this is her first litter. I hope. I heard the first ones are really small… will it be ok if I leave the boy babies in with the dad annd the girl babies in with the mom? pls help me!!!