Hi, There are many SSR types out there. A common one is about 5 by 6 cm and looks like this:
Some How-to about controlling power with Arduino is on the http://ArduinoInfo.Info WIKI See:
http://arduino-info.wikispaces.com/ArduinoPower
There are also boards with several lower-power SSR's like this one: http://goo.gl/9o5IQ
What are you trying to control??
DISCLAIMER: Mentioned stuff from my own shop...