I normally hand-solder 0805 parts and I also have done 0603 parts. They can share each other's pads. An 0805 part on an 0603 pad isn't very happy and it's not a saleable product but it does work. 0603 goes on to 0805 pads nicely.
The images of the part footprints you are seeing in Eagle are just a representation. It's not exactly that size. Just put down the pads in the manufacturer's recommended places and it will totally work. For standard size resistors, there's no need to even check the datasheets.
Adafruit sell a couple of nice 'books' of SMD parts. Buy the 0805 book and also a blank book to store all your specialised parts in.
Once you start soldering SMD with a stencil, then you will start avoiding through-hole parts and favoring SMD everywhere. I would try to do your board with all components SMD, except the connectors. When you see the parts magically align themselves to their correct places on the board, you will be hooked on this contsruction method.