Document DOTNET_Thread_DefaultStackSize environment variable and runtimeconfig.json property for .NET 10

docs/core/runtime-config/threading.md

@@ -2,6 +2,7 @@
 title: Threading config settings
 description: Learn about the settings that configure threading for .NET apps.
 ms.date: 11/04/2021
+ai-usage: ai-assisted
 ---
 # Runtime configuration options for threading
 
@@ -265,3 +266,44 @@ Project file:
 
 </Project>
 ```
+
+## Default stack size for new threads
+
+- Specifies the stack size (in bytes) for new VM threads that are created with the default stack size.
+- This setting allows configuration of the stack size for threads created by <xref:System.Threading.Thread> and other APIs that do not manually specify a stack size.
+- If not set, the stack size is determined by the system or runtime default.
+
+| | Setting name | Values | Version introduced |
+| - | - | - | - |
+| **Environment variable** | `DOTNET_Thread_DefaultStackSize` | An integer value (bytes). Example: `1048576` for 1 MB | .NET 9 |
+
+### Examples
+
+Set the stack size to 2 MB for all new threads:
+
+```shell
+# Windows
+set DOTNET_Thread_DefaultStackSize=2097152
+
+# PowerShell
+$env:DOTNET_Thread_DefaultStackSize="2097152"
+
+# Unix
+export DOTNET_Thread_DefaultStackSize=2097152
+```
+
+Then run your application:
+
+```shell
+dotnet myapp.dll
+```
+
+#### In C# code (for reference, no runtimeconfig property):
+
+```csharp
+// Stack size is determined by the DOTNET_Thread_DefaultStackSize environment variable
+var thread = new Thread(() => { /* thread logic */ }); 
+thread.Start();
+```
+
+**Note:** This setting does not affect threads for which a non-default stack size is specified at creation.