I can't find these details in documentation. Any suggestions where to find it.
The memory requirements of Wildfly are almost completely dependent on your applications and workloads. You need to deploy your applications and do realistic load testing that reflects what will happen in production. Asking this is like asking "How much memory does my operating system need?" - it's not possible to answer without specific knowledge of what you are doing with it.
On the minimum side, I can start a default standalone instance with nothing deployed on JDK 8 with a 24mb heap size (which uses around 40mb of metaspace too) although it starts slowly due to GC. Obviously that will fail due to a OutOfMemoryError as soon as you deploy an application, load the admin console, or actually do anything with it, so although that is a "minimum heap size", it's not something you could actually use.
On the maximum side, there is no real limit except that imposed by your JDK and operating system. However as your heap size gets larger, GC may become more of an issue so you will need to carefully review your GC settings. I know of people who run EAP 6 with heap sizes in the hundreds of gigabytes, so WildFly 8 should be similar.