Concerning the size, the 2 phones are approximately the same: Blackberry has 120 x 62 x 11.5 mm and weights 135 g, and Sony Ericsson has 92 x 55 x 16.5 mm and weights 110.8 g. Resulting that Blackberry is heavier, but thicker. 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 3 inch LED-backlit LCD, capacitive touchscreen, 16M colors with 320 x 480 pixels that Sony Ericsson has. But, Blackberry has a bigger resolution and a larger display.
The advantage for Blackberry is the internal memory, being equipped with a 4 GB storage with 768 MB RAM, which is bigger then Sony Ericsson's 1 GB (320 MB user available) and 512 MB RAM. The rest of the features are approximately the same, except Blackberry looses ground on the operating system part, because Sony Ericsson is equipped with Android Gingerbread.
The list can go on, but util Blackberry Torch 9850 will be available, the comparison with Sony Ericsson Xperia Active is only a technical one.