iPhone Case Size Guide
Want to double check which phone model you have?
Here's a quick guide to help figure out which phone you (or someone else) has:
Below are photos to help show the difference between the different iPhone models:

iPhone 13 Mini
The iPhone 13 Mini has a 5.4" screen (diagonally measured as shown above).

iPhone 13/13 Pro
The iPhone 13/13 Pro has a 6.1" screen (diagonally measured as shown above).

iPhone 13 Pro Max
The iPhone 13 Pro Max has a 6.7" screen (diagonally measured as shown above).

iPhone 12 Mini
The iPhone 12 Mini has a 5.4" screen (diagonally measured as shown above).

iPhone 12/12 Pro
The iPhone 12/12 Pro has a 6.1" screen (diagonally measured as shown above).

iPhone 12 Pro Max
The iPhone 12 Pro Max has a 6.7" screen (diagonally measured as shown above).

iPhone 11
The iPhone 11 has a 6.1" screen (diagonally measured as shown above).
iPhone XR
The iPhone Xr has a 6.1" screen (diagonally measured as shown above). It's important not to confuse this model with the models above.
iPhone 7, 8, and SE
The iPhone 7, 8, and SE (2020) are the same size. These model iPhones have a 4.7" screen (diagonally measured as shown above).
These two models are different than the iPhone 6/6s model Apple offers due to no longer having a headphone jack and a different camera design. If you currently have an iPhone 6, you'll want to upgrade to the iPhone 7, 8, and SE cases to have the right fit for to these upgraded features :)

iPhone 7 Plus & 8 Plus
These model iPhones are the larger iPhone models Apple offers. Both models have a 5.5" screen (diagonally measured as shown above).
These two models are different than the iPhone 6 Plus / 6s Plus model Apple offers due to no longer having a headphone jack and a different camera design. If you currently have an iPhone 6 Plus or 6s Plus, you'll want to upgrade to the iPhone 7 Plus & 8 Plus to have the right fit for to these upgraded features :)

iPhone 6 & 6s
The iPhone 6 and 6s are the smaller iPhone 6 models Apple offers. Both of these models have the 4.7" screen (diagonally measured as shown above).
It's important to note that the 's' version is very slightly larger.

iPhone 6 Plus & 6s Plus
The iPhone 6 Plus and 6s Plus are the larger iPhone 6 models Apple offers. Both of these models have the larger 5.5" screen (diagonally measured as shown above).
The same as above, the 's' version is very slightly larger.

iPhone 5/SE
The iPhone 5/SE is the smallest iPhone Apple offers. This model has a 4" screen (diagonally measured as shown above).
The iPhone 5 and the iPhone SE have the same size dimensions overall, so the iPhone 5/SE cases will work for both models.