Hello, you can do so with a RenderTarget2D.
1) Create a 50x50 RenderTarget,
2) use GraphicsDevice.SetRendertarget to send all your drawing to said rendertarget.
4) SpriteBatch.Draw (using as destination rectangle the created rendertarget .Bounds),
6) use GraphicsDevice.SetRendertarget(null) to reset all the drawing to the BackBuffer.
In the end your rendertarget will have the scaled texture, with the quality of the scaling depending on your TextureSampling system.