You can't possibly expect anyone to know what size stem you have just by saying you have a 80s schwinn sprint. More than likely it will be 25.4 but there's no way anyone can be 100% sure without as much as a picture or even better a measurement!
A set of inexpensive (measuring) calipers are a great tool to have in your box. I use mine way more than I expected, but I've got a lot of bikes, and I make a lot of changes. You can measure inner dimensions (the hole in your stem), or the outer dimension of your current handlebars.