Is there any kind of label sheet size or template that will fit on a piano keyboard? I want to print from my printer ABC's that will fit on the keyboard. Thanks!
Office stationery suppliers sell label sheets in a huge variety of formats, including labels 10mm deep which should be small enough to fit on standard piano keys if you use them "sideways". But even a single sheet would probably label three 88-note keyboards, so you may have to buy far more labels than you actually need.
Google for office suppliers - the SE rules say that we can't give specific product recommendations here.
Here's a template: Piano Keyboard Diagrams to Print Out.
You could also buy the stickers which are widely available. For example, Keysies Transparent Plastic Removable Piano and Keyboard Note Stickers