Is it possible to obtain something with a screen about postcard-sized that can either plug into my Canon EOS 20D camera and act as a larger screen, or alternatively will accept the CF card from the camera via a card reader, to check for focus accuracy. I dont want to be carrying my laptop miles from the car.
Well, Canon 20D doesn't have live view and that makes it a little hard but I guess it shouldn't be that hard to hook a bigger monitor to it.
I just looked up the B&H quickly and I found this portable monitor which has analog inputs:
Vello RigVision 9" HD Camera Monitor
It has many different inputs and I guess you should be able to connect to your camera.
You can find many other similar products with analog input here on B&H.
Also, because Epson P-3000 is discontinued, you can check other similar products, such as Sanho HyperDrive multimedia player and storage device.
The EPSON P3000 is probably what you are looking for. It also does backups too as a bonus. It has a 4" LCD and can show both JPEG and RAW images from a Compact-Flash card. You can zoom into images to check focus.
Just note that because it is hard-drive based, it does not work above 10,000' of altitude which can be a concern for some and irrelevant for others. I have not tried but the problem would go away if you manage to open it and substitute an SSD inside.
It seems like there are a number of smartphone and tablet options (none of which I've needed to try personally, but they may give you some ideas):
use an iPad with a CF card adapter
as above, use an appropriate adapter to record onto SD or Micro-SD cards which can be read by many Android tablets