Concerning the size, the 2 phones are approximately the same: Blackberry has 120 x 62 x 11.5 mm and weights 135 g, and Google Nexus has 123.9 x 63 x 10.9 mm and weights 129 g. Resulting that Blackberry is heavier, and smaller. Regarding the display, Blackberry is equipped with a 3 inch TFT capacitive touchscreen, 16M colors with 480 x 800 pixels, which is kind of weaker compared to the 4 inch Super AMOLED capacitive touchscreen, 16M colors with 480 x 800 pixels that Google Nexus has.
The advantage for Blackberry is the internal memory, being equipped with a 4 GB storage with 768 MB RAM, which is bigger then Google Nexus's 16GB storage and 512 MB RAM. The rest of the features are approximately the same, except Blackberry looses ground on the operating system part, because Google Nexus is equipped with Android Gingerbread.
The list can go on, but util Blackberry Torch 9860 will be available, the comparison with Samsung Google Nexus S is only a technical one.