Resize media image stream in Sitecore

ResizeImageStream.cs

var mediaItem = GetMediaItemFromSomewhere();
var mediaOptions = new global::Sitecore.Resources.Media.MediaOptions() { MaxWidth = 300 };
var pipelineArgs = new global::Sitecore.Resources.Media.GetMediaStreamPipelineArgs(new global::Sitecore.Resources.Media.MediaData(mediaItem), mediaOptions);

// Run resizing pipeline
var resizeProcessor = new global::Sitecore.Resources.Media.ResizeProcessor();
resizeProcessor.Process(pipelineArgs);

// Copy resized OutputStream to MemoryStream, or use OutputStream.Stream directly
var memoryStream = new System.IO.MemoryStream();
pipelineArgs.OutputStream.CopyTo(memoryStream);
pipelineArgs.OutputStream.Dispose();
memoryStream.Seek(0, System.IO.SeekOrigin.Begin);