I haven't really decided on a specific 7 segment display yet, but most of the larger displays I see are 7-12 volt. I'm not really set on a certain type or size just yet.
So far I've seen displays like this one:
http://www.ebay.com/itm/4-inch-7-segment-RED-LED-display-common-anode-1pcs-/360608554918?pt=LH_DefaultDomain_0&hash=item53f5f1dfa6
Voltage:10V~12V Current:160mA
But maybe eBay isn't the best choice? I don't see many specs on that display (like brightness). I want a bright display for outdoor viewing in the sun.
I've seen some other displays on Digikey
6.8V 10mA
or
Mouser
8V 30mA
I've also seen some that are only 1.85V 30mA (although not in stock)
So far, I've been working on a small prototype on my breadboard. Once I get the software to work how I want it, then I'll start making a full size version. I've been trying to read a lot and see what others have done before, but lots of projects look to be unique. Just trying to figure out all of the different options out there for what will work best for me. Daunting task for a non-electronics guy.
I'll PM you later for the component for expressPCB software. Thanks!
(I'm still amazed at how easy it is to program something on the Arduino and have it do things. Never thought it would be this easy.)