Upload & Sell: Off
There is no standard ‘digital’ print size. If you go for the best, pigment ink on high end archival paper, then in the US, the paper sizes are based on office sized paper: 8.5 x 11, 11 x 14, 11 x 17, and 11 x 22 inches. There is also the odd 13 x 19 inch paper size.
The standard dSLR aspect ratio is 3:2.
‘Standard’ frame sizes are based on century old larger format sizes of 4 x 5, 8 x 10, etc. The frames don’t fit paper sizes. In short, it is a mess.
I would look around at local framers and see what they will charge. Or look at online custom frames, for example: frameDestination.com.
The rest of the world have it a little better (but only a little) in that international paper sizes are closer to a 3:2 aspect ratio than archaic US sizes.