There are all sorts of home theater calculators out there that will tell me all sorts of things about distance and screen size and viewing angles, but none seem to be getting me the particular information I'm looking for. Let me see if I can explain:

We currently own an aging RPLCD TV. Because it's so deep, it sits on a stand. This arrangement isn't great for having a baby/toddler around, so we'd like to replace this one with a flat screen, thus entering 2009.

What I want to find out is how large a screen I'll need, when mounted on the wall, to match the area of my vision that the current TV fills. The old TV is on a stand and sits roughly 3' away from the wall. The new TV would presumably be no more than about 3-6" from the wall, meaning a difference in distance of around 2.5' or more.

Now, if I got the same screen size and placed it 2.5' further back, I'd assume that the new TV is going to look smaller. I don't know if it'll look a lot smaller or not, but I'd think it would. What I don't know how to do - because my geometry skills have around 18 years of rust on them - is how to figure out how much smaller the new set would look.

Lets figure the following numbers: Old TV is a 60" screen, viewing distance from directly in front is approximately 13'. How do I figure out what size TV will match the area the old set takes up in my field of vision?

ps-I recognize that I could be perfectly happy with a 60" flat screen, but I like big TVs and I'm looking for some data to use as justification with the wife