Need to change my print size on my C6280 Photosmart All-in-one printer, make it larger

It depends on what you are printing, but generally this needs to be adjusted in the software you are using.
If you print pictures or photos, you have to find an option to scale the printout up, either during printing, or beforehand.
If you print PDF documents, in the Print dialog box there is an option to "fit printer margins" or "auto-adjust to printer size" or something to that effect in every PDF reader application. If however the document is formatted for your paper size it will not be scaled, and if it's formatted for an even bigger sheet size, it will scale down the printout.
If you print documents from Word, there's no easy way to make the printout bigger - you can change the font size to a larger one in the whole text, but chances are, that this will ruin the text formatting.
In Excel there is an option to scale everything by a given percentage, which will automatically tile the table into as many sheets as necessary. and then the area will be printed on as many paper sheets as necessary to get
There's no one solution, unless your printer driver has an option to do a fixed scaling - but this will usually be sensible only for enlarging printouts that occupy a smaller area on paper, and you want them to use the maximum paper area. I'm sorry, but you have to experiment a bit to get the hang of it.
Good luck!