I use a Toro EZ flow 2604 valve for my drip line without any problems. You basically need a valve that can handle low flow and pressure so that the valve can open/close properly. The 2604 has a Flow range of 0.25-30 GPM and an Operating pressure of 10-150 PSI which should work well for most drip lines.
You can use anything from 1/4" polytube all the way to 1" polytube. Most homeowners will use 1/2" polytube as a mainline but it really depends on how much water you need to irrigate. Since you already have poly laid I would stick to that and transition that to the ½ and ¼ drip tubes as needed.
Look over some of the drip tutorials; they were very helpful for me when I planned my drip system.
http://www.dripirrigation.com/drip_tutorial.php
http://www.irrigationtutorials.com/dripguide.htm