The size of the microwave you should choose depends on your specific needs and the available space in your kitchen. Here are some factors to consider when determining the right microwave size for your situation:

1. Available Space: Measure the space where you plan to place the microwave. Make sure to account for both the width and height of the area. Microwaves come in various sizes, including compact countertop models, over-the-range microwaves, and built-in models, so choose one that fits your available space.
2. Cooking Needs: Consider the types of dishes you typically prepare in the microwave. If you often heat or cook large dishes or use microwave-safe cookware that may be taller, you'll need a microwave with a larger internal capacity.
3. Family Size: If you have a large family or frequently entertain guests, a larger microwave with more capacity might be a better choice to accommodate larger quantities of food.
4. Cooking Styles: If you use the microwave primarily for reheating, defrosting, and simple cooking tasks, a smaller microwave should suffice. If you plan to use it for more complex cooking tasks, such as baking or roasting, you might want a microwave with convection capabilities and more space.
5. Wattage: Microwave wattage can vary, and higher-wattage microwaves can cook food faster. If you want quicker cooking times, a higher-wattage microwave might be a good choice.
6. Budget: Your budget is also a significant factor. Larger and more powerful microwaves tend to be more expensive. Make sure the size and features of the microwave align with your budget.
7. Features: Consider what features are important to you, such as sensor cooking, convection cooking, preset cooking programs, and more. Larger microwaves often come with a wider range of features.
8. Placement: Think about where you plan to place the microwave. Countertop models are versatile but take up counter space. Over-the-range microwaves save counter space but require specific installation. Built-in models can be integrated into your kitchen cabinetry.

In summary, choose a microwave size that suits your cooking needs, kitchen space, and budget. The ideal microwave size will vary from one person or household to another, so consider these factors to make the best choice for your specific situation.