Language
Name
Lennie
Imagery
Animals
"He drank from long gulps, snorting into the water like a horse" - Simile
"and he walked heavily, dragging his feet a little, the way a bear drags his paws".
Strong as a bull
Both bulls and horses are strong farm animals used to perform simple domestic tasks such as carrying big, heavy items such as carts. 
Both bulls and horses are strong farm animals used to perform simple domestic tasks such as carrying big, heavy items such as cart 
Could mean that George is trying to use him in order to do the bulk of the work on the ranch, linking back to the recurring theme of loneliness; they were purely friends because of business
Also possesses a wild, an uncontrollable side, whereby he is not able to have a sense of self-control for his actions. Emphasised by his killing of the numerous rabbits as he claims to "pet" them and his killing of two females, one of which is Curley's wife. 
"Small"
Could connote many things , one of which is the irony of him being big and strong, yet still being called "small".
Could imply his role and how other characters treat him, as they treat him with disregard and disrespect despite the fact that he is much bigger and stronger than the rest. Could be Steinbeck's method of passing across the message that it is not always the strongest person in a place that leads. Could also be his method of conveying the fact that one should not judge a book by its cover.
Symbolic of his mental ability and intelligence
Germanic meaning for "of lion's strength"
"Lennie squirmed under the look(from Curley) and shifted his feet nervously"
Links back to the fact that he is quite timid and fearful, doesn't act his own size - again links to the them of fate in that because he is "Small", he must act timid.
Form
The form of the prose, which is a mini novel known as a novella, is used to reiterate the theme of fate. Throughout the novella, we do not see Lennie or any other character change in any way, which shows that because of their double meaning names, they cannot change or adapt in any way, because of the shortness of the prose.