One of the annoying things about WhatsApp is its focus almost exclusively on phones: the desktop Mac app is a very recent development – it only arrived this August – and there's still no sign of an official app for the iPad. But a new beta of the iOS app means you can get a full-screen WhatsApp app instead of running the iPhone app in single or double-sized views.
The trick was spotted by WaBetaInfo, and to use it you'll need to install two things: TestFlight, which is Apple's app for running apps that aren't yet available in the App Store, and the latest WhatsApp beta for iOS. It has to be the beta: the feature isn't in the standard iOS app yet. And you'll need to be enrolled for the beta already, because the current beta programme is now full and isn't currently accepting new members.
Here's how to do it.
How to get WhatsApp for iPad
First of all, you'll need to have TestFlight installed on your iPhone and have access to the WhatsApp For iOS Official Beta Program. If you do, you can then install the beta iOS version of the app through TestFlight on your iPad too. There's no way around this: if you haven't signed up for beta access, there's currently no way to run WhatsApp as an iPad app instead of as an iPhone app.
Once you've got the beta installed on both devices, open the WhatsApp beta on your iPhone, sign in and go into WhatsApp Settings > Linked Devices. This will generate a QR code you can then scan on your iPad in order to link it to your WhatsApp account. You'll need to use the same WhatsApp account on both your iPhone and your iPad.
The reason this beta now works on iPad is because it uses what's called Companion Mode, which is a way of using more than one device for the same account. Any messages you send or receive on your iPad will be mirrored on your iPhone and vice-versa, and your messages and calls will be protected with the same end-to-end encryption.
If you don't already have access to the beta programme, don't worry: if the new feature is now in the beta, it shouldn't be long before it makes its way into the final release. WhatsApp's development tends to move very quickly so iPad compatibility should be imminent.