My aunt has the Rebel XT which is 8mp and she gets fantastic prints at 11x14.you divide the DPI by of the resolution of the photo.
take the highest resolution of the 20d is 3504 x 2332 and divide that by 300 DPI that would be 11.68 x 7.73, Now you can lower the DPI and get bigger prints but the qaulity/sharpness of the print won't be as high.
hope that helps...
Of course you could, You can get a big blurry image 12 feet wide if you want. Photoshop can scale imaegs but it can not invent new detail that is not recored by the camera.If it supports RAW images then you could theoreticaly go much bigger.
With RAW proccessing you could scale the image up a Huge amount in Photoshop and go larger than 24 x 30 if you wanted.