For 5 gal I wouldn't go smaller than 3/8'' tubing. If your batch size is 3 gal or smaller you could get away with 1/4''.
And just so you know if you calculate the surface areas based on manufacturers tubing OD you will get the following numbers:1/4'' = 14.35 square inches/foot, 3/8'' = 19.1 square inches/foot, 1/2'' = 23.8 square inches/foot. So the larger the tubing the more surface contact you will get. Also, you have to look at the temp difference in your water vs. how much your pushing through your chiller. If you don't have enough flow your water will saturate with heat before exiting the chiller, reducing your efficiency and extending the time necessary to chill the wort.
This is why 3/8'' is a generally accepted size for 5 gal and 1/2'' is a generally accepted size for 10 gal.