There is actually another quote where Slim is introduced to us in Chapter 2. Whilst George and Lennie are in the bunkerhouse, Slim enters:
A tall man stood in the doorway. He held a crushed Stetson hat under his arm while he combed his long, black, damp hair straight back. Like the others he wore blue jeans and a short denim jacket. When he had finished combing his hair he moved into the room, and he moved with a kajesty only achieved by royalty and master craftsmen. He was a jerkline skinner, th eprince of the ranch, capable of driving ten, sixteen, even twenty mules with a single line to the leaders... There was a graviy in his manner and a quiet so profound that all talk stopped when he spoke. His authority was so great that his word was taken on any subject, be it politics or love.
Here, then, you have an excellent description of Slim. Note how he is different to the other characters, particularly Curly. He has natural authority and confidence and does not need to bully or swagger. He is also very good at what he does and is respected by all other characters. He is very kind and perceptive - note how it is he who tries to comfort George at the end of the novel whilst the other characters walk off. And finally, he is a doer instead of a dreamer - he takes great pride in the high quality of his work and gains respect as a result.