How many gallons are you chilling? That should work fine with a partial boil. For a 5 gallon batch it may be too small and for 10 gallon batches it is way too small. I priced out all my supplies at my local Home Depot and found for just a few dollars more you can buy a chiller from these guys in New York for about the same price. http://www.nybrewsupply.com/products/wort-chiller-copper-immersion.php#c1250
I was looking around the web the other day trying to find the specs on length to build an immersion chiller. Seems like all the DIY instructions used 50' length of tubing. 20' will still work it's just going to take longer to chill your wort.
as hot as it has been in Georgia lately, I have been using a 50 ft immersion chiller plus a 25 foot prechiller. I don't ice down the prechiller until the wort is below 100. My tap water the other night was 78 degrees.
edit: I should have said I guess it depends on how cold your tap water is. My 25 foot worked ok in the winter time when the tap water was below maybe 50 degrees (don't remember)