someone may correct me on this, but I think that because your computer at home is possibly only linked to a printer capable of A4, that the option to print on A3 might not appear.
Your daughter, I think, would have to produce the work on a document which is A3 (maybe you have an old A3 template document somewhere on your Hard Drive), then take it on pen-drive to Staples.
Failing that, she may have to do it on A4, and ask Staples if she can transfer it to A3 at their store, but she would have to adjust the document for A3 while with them. Not sure if they would let her do this, or whether they would do it for her. Sorry this is a bit lengthy.