There are a few factors to consider when choosing the size of trash bags:

1. The size of your trash bin: You'll want to choose a trash bag that fits your trash bin snugly, so that it doesn't fall into the bin or spill over the sides. Measure the dimensions of your trash bin and compare them to the size of the trash bags to ensure a good fit.

2. The amount of trash you generate: Consider the amount of trash you typically generate in a week or month. If you generate a lot of trash, you'll want to choose larger trash bags to accommodate all of the waste. On the other hand, if you generate a smaller amount of trash, you may be able to get away with using smaller bags.

3. The type of trash: Different types of trash may require different sizes of trash bags. For example, you may need larger bags for heavy items like broken furniture or appliances, or smaller bags for items like food waste.

4. Personal preference: Ultimately, the size of trash bags you choose will depend on your personal preferences and needs. Some people may prefer larger bags for convenience, while others may prefer smaller bags to reduce the amount of waste they generate.

In general, it's a good idea to have a variety of trash bag sizes on hand to accommodate different needs and situations. This will allow you to choose the right size for each task and ensure that you have enough bags to last for a while.