Here are the size classifications for eggs:
1 jumbo egg - 2.50 ounces
1 extra-large egg - 2.25 ounces
1 large egg - 2 ounces
1 medium egg - 1.75 ounces
1 small egg - 1.50 ounces
Depending on the bantam, her eggs may be in the small range, or she may lay eggs that are tiny, 1 to 1.25 ounces or even smaller. When a recipe calls for an egg without giving a specific size (as most do), it is calling for a large egg (2 ounces or so). So, if you have a kitchen or egg scale and can weigh your eggs, you will be able to determine exactly how many of your smaller eggs you need to use in a recipe calling for large eggs. If you don't have a kitchen scale, you will simply have to estimate.
Keep in mind that there is usually a higher yolk-to-white ratio in bantam eggs, so when baking you may still see some variation in your results.