WhatsApp is a cross-platform messaging app that allows users to communicate through login data or Wi-Fi, without having to pay for text messages. If you find the font size too big or too small, you can change it as needed. If you are using iOS, you need to use your iOS settings to make the changes, while Android users can change the font size within the app itself.
Steps
Method 1 of 2: iOS

Step 1. Open the Settings app
It is no longer possible to change the font size within the WhatsApp application. Instead, you'll use iOS built-in font options to change the size.

Step 2. Select Chat Settings
If you're using iOS 7, select General instead.

Step 3. Tap the Text Size option

Step 4. Drag the slider to adjust the text size
Dragging it to the left will make the font size in WhatsApp smaller and sliding it to the right will make it bigger.

Step 5. Open Settings → General → Accessibility → Larger Text to enable Larger Size
This is useful if you have difficulty reading small print.
Method 2 of 2: Android

Step 1. Open the WhatsApp application
You can change the font size on WhatsApp within WhatsApp settings.

Step 2. Press the menu button (⋮) and select Settings
It's in the upper right corner.

Step 3. Tap Chat Settings

Step 4. Tap Font Size and select the size you want
There are three options you can choose from and the Medium option is the default size.