Go to EveryMac.com's Ultimate Mac Lookup, enter one of the listed iDevice numbers or identifiers, hit lookup and you will get the RAM size.

Please check the *: Apple provides no official details regarding RAM used by the %iDevice%...

Various sites compile nice single page summaries. The RAM across devices is listed just below the top horizontal line:

• http://iossupportmatrix.com

This works in Swift 4.

The really important difference here is: It casts to Int64 before multiplying, because otherwise you get quickly overflows, especially if you run it in a simulator where it uses the PC Memory.

var pagesize: vm_size_t = 0

let host_port: mach_port_t = mach_host_self()
var host_size: mach_msg_type_number_t = mach_msg_type_number_t(MemoryLayout<vm_statistics_data_t>.stride / MemoryLayout<integer_t>.stride)
host_page_size(host_port, &pagesize)

var vm_stat: vm_statistics = vm_statistics_data_t()
withUnsafeMutablePointer(to: &vm_stat) { (vmStatPointer) -> Void in
    vmStatPointer.withMemoryRebound(to: integer_t.self, capacity: Int(host_size)) {
        if (host_statistics(host_port, HOST_VM_INFO, $0, &host_size) != KERN_SUCCESS) {
            NSLog("Error: Failed to fetch vm statistics")
        }
    }
}

/* Stats in bytes */
let mem_used: Int64 = Int64(vm_stat.active_count +
        vm_stat.inactive_count +
        vm_stat.wire_count) * Int64(pagesize)
let mem_free: Int64 = Int64(vm_stat.free_count) * Int64(pagesize)