Depends on how elaborate a sled you want to make. You can go with a one-runner simple fence and panel; a two runner with fences front and back with a safety guard.
Here's a link to a fancy sled - the video pertains to the 5-cut method (I'm not advocating this method - I'm only providing the link) but on the web page he lists the dimensions of the sled/runners etc. 5 Cuts To A ?Perfect? Cross Cut Sled | William Ng School of Fine Woodworking