You can add a monitor to your case in the 5-7" size range. but for general work you should not really go below 14.
On the subject of adding monitors to you case there was a artical in PCF about moding a screen from the game cube (I think that's right) to add to your case, cheaper than buying a whole screen when one for 30quid from ebay will do
Hey...LOL, I just bought the 14" from geeks.com and it's pretty nice. For basic use, it'll work. I was gonna build a cheapo pc to sell and include the 14" but my bro has been wanting a flat screen..so maybe I'll just sell it to him. I paid $120 shipped.
No 5-7 inches are not touch screens, to make a monitor touch screen it requires a matrix, Small touch screens like the type on PDA's normaly require a pointer device (stylus) that has a small transmitter in it.