The smallest tank dedicated tank chiller I know of is by Prime and for a 185g, it cost about $650.
Make your own for a lot less. for a 10g-20g get a 12V mini-fridge like this one
http://www.amazon.com/exec/obidos/tg/detail/-/B00009IM01/002-6565075-2918404?v=glance
it runs about $20-$30 (the spiderman ones are cheaper than the white ones), tubing a small pump, grommets, and an inverter will run about $20 more. When you don't need it as a chiller pull the tubing out, cork the two holes, and use it in you car or at your desk.
You could actually place a small pump in the tank and conceal it but it doesn't look as nice.
Oh, in size it is about 11" talland 8" wide. It just holds a six pack of soda cans.