I am a fairly big point guard (5'11/195 lbs) and i would say that i am pretty quick but i always have a hard time when i have to face a shorter/smaller player than me. I don't know how to be most effective against them because somehow they always shut me down when i am on offense and they are difficult to guard on defense. The bigger problem for me though is getting my offense right against them, the only advantage i can consistenly use is shooting over them.
Get used to that whatever player you are guarding your goal is to try to get him out of the racket and if possible get him on the 3 point line. Have your one arm facing the way you want him to go and your other facing above him to deny any overhead pass or shoot. Always remember that you must make him go the way you want for example, make him go left because a lot of your teammates are there and he want be able to drive. Don't stand square at him, because then, he will just drive at any side he wants and score the basket or assist a basket. Also if you fear that he will score you won't be able to play him. Get in the court with confidence, observe the game, and co-operate with your teammates!!