There isn’t a set scale your stop motion film, sets or characters have to be. It’s one of the great things about the stop motion medium, you’re free to create whatever you want. With that in mind, there is an important relationship related to scale that you should keep in mind. Scale is related to detail and cost. Generally speaking, the larger something is the more detail it will have. The more detail something has the more expensive, both in time and money, it will cost to produce. A house constructed at a small scale like 1:48, also known os O Scale or Lego Scale, may not need actual slats for the house’s siding where a house done a 1:12, also know as one-inch scale or action figure scale, may have siding, roof tiles, moulding and lots of other architectural details. In theory the small house would cost less to produce than the big house, but again, this is a just a theory. You can choose to omit details from large scale set pieces and you can choose to meticulously add detail to small scale items. It’s up to you, or is it?
Available space plays an important role in determining what scale you will work as does the type of camera shots you require. If you need a long dolly shot of a roadway filled with cars but only have a small closet worth of space to work with you will probably need to work at smaller scales. In the same closet, if you have an interior single room shot with a single character you can get away with making the scale bigger. When planning for your production it’s important to visualize and test your set in the space you have and determine scale based on what fits and is comfortable to work with.
In short, you should work at a scale that fits your space, budget and required level of detail. This will probably be somewhere in the 1:12 to 1:48. For general reference a Barbie doll is roughly 1:6, normal action figures are 1:12, green army men are 1:32 and lego minifigs are 1:48.