To determine which size iPhone you have, you can follow these steps:

1. Check the Model Number: Go to the "Settings" app on your iPhone. Then tap on "General" and select "About." Look for the "Model" field. You'll see a model number starting with "A." Note down this number.
   
   
   
2. Identify the Model Number: Once you have the model number, you can look it up online to find out which iPhone model it corresponds to. Websites like Apple's support page or third-party sites can help you identify the specific iPhone model.
   
   
3. Compare Size Options: Once you know your iPhone model, you can compare its physical dimensions with those of different iPhone models to determine its size. You can find the dimensions of each iPhone model on Apple's website or through various other sources online.
   
   
4. Physical Comparison: If you have other iPhones or know someone who does, you can physically compare the size of your iPhone with theirs to determine which size it is.
   
   

Remember, iPhone sizes typically refer to the physical dimensions of the device, such as screen size and overall form factor (e.g., iPhone 12 Mini, iPhone 12 Pro Max).

You can buy the best iPhone case by following steps.

1. Determine the size and model of your cell phone. As we mentioned above, different models of cell phones have slightly different size design, edge design, etc. If you buy directly without knowing it well, there may be incompatible problems.
   
   
2. Function. This depends on personal needs. For example, some cell phone cases support the function of wireless charging, or have a stand, etc., choose according to your needs.
   
   
3. style. There are minimalist styles, business styles for work occasions, pink and cute women's phone cases and so on, all of which have a variety of options to meet different tastes.
   
   
4. Material. Now the most popular market is PC material, PC materials are relatively hard and easy to scratch. With the passage of time, especially in ultraviolet radiation or exposure to harsh environmental conditions, accelerated aging will lead to yellowing, brittle or loss of elasticity of the phone case. It is best to use materials such as silicone, TPU (thermoplastic polyurethane), polycarbonate, leather and metal, and read the product's manual before purchasing.
   
   
5. Price: There are many prices for cell phone cases on the market, but keep in mind that higher-priced cases may offer advanced features or use premium materials, and there are also many affordable options if you just want basic protection.
   
   
6. Brand: go through the customer reviews before buying to determine if the seller is trustworthy. I would recommend TORRAS more, we sell cases for all models of iPhone 15 series, iPhone 14 series. These cases are made of TPE + Acrylic, which can ensure the case is drop and scratch resistant, and at the same time, it is also guaranteed not to oxidize. Many customers have given positive feedback.

The fit of an iPhone 14 case on an iPhone 13 would likely be imperfect. Although iPhones within a series often share similarities in size and shape, there could be slight variations between models, particularly in dimensions, button placements, and camera configurations. Apple occasionally makes subtle adjustments with each new iPhone release, even if the overall form factor remains similar.

Using a case designed for the specific iPhone model ensures the best fit, protection, and access to ports and buttons. While some cases might fit across similar iPhone models, it's not guaranteed, and it's advisable to use a case intended for your iPhone 13 to ensure optimal compatibility and protection.