You'll be limited by the size of holesaw you can find. And make a trial one - don't rely on measurements.
It only makes sense once you've done one. ;-)
If you're going to buy a hole saw, find one as close as you can - but remember the plug won't be the same size as the cutter says. Hence doing a trial one. You can buy proper cutters designed to make plugs of a certain size - but one that size if you could find one - pricey.
If you sand both the door and the donor plank, you can likely find a reasonable match grain wise - if you're lucky. I had a couple of doors I wanted to leave as oiled wood and spent some time patching them. Great fun if you have the time - like we all probably have now. And mixing stain to get the colour as near right as possible. Not an invisible repair, but looks good and tells the history in an old house.