You need an Android tablet/phone that runs Android 4.1 or later.
Pattern Keeper is supported on Android devices running Android version 4.1 or later and has the Google Play Store installed. However, it performs more smoothly on newer devices. If buying a new device to run Pattern Keeper on then it is recommended to get at least 2GB RAM.
If possible also avoid Android 8.1 as that version of Android is known to be a bit unstable.
There is no requirement for screen size and you can run it for example on a phone. Some people prefer the bigger screens though as you can see more of the chart at once.
Since Pattern Keeper is currently only available on the Google Play store it is not possible to install it on Huawei devices and Kindle/Fire tablets without Google Play Store. A bit of Googling will tell you how to install Google play on a tablet that does not have it but since that can void the warranty it is not something we officially recommend.