To find the answer to this question you need go no further than the first chapter and the part when Boxer and Clover are introduced. This clearly presents us with a good image of Boxer as a horse and also gives us other details about his characteristics:
The two cart-horses, Boxer and Clover, came in together, walking very slowly and setting down their vast hairy hoofs with great care lest there should be some small animal concealed in the straw... Boxer was an enormous beast, nearly eighteen hands high, and as strong as any two ordinary horses put together. A white stripe down his nose gave him a somewhat stupid appearance, and in fact he was not of first-rate intelligence, but he was universally respected for his steadiness of character and tremendous powers of work.
Note how the description given of Boxer emphasises his strength and size. His hoofs are described as "vast" and he himself is described as an "enormous beast" and possessing great strength - in fact double that of most horses. His capabilities of work are likewise described as "tremendous". Clearly Orwell presents Boxer as a very strong horse who is able to work incredibly hard. Of course, it is important not to forget his limitation - his lack of intelligence, which is a key factor that is exploited by the pigs in their tyrannical rise to power.