Mtn Bike size translate to road bike?

I'm looking to get a road bike and wondering how my 15"
mountain size translates to a road bike?

 

Welcome to the forum, hemanjr!
Why make things complicated? Goto a reputable bike shop and have them measure you. These guys do this type of thing regulary and will give you sound advice.
If there's no bike shop in your area, you can have it done online for a reasonable fee:

Go here: Bio-Racer Bike Setup

 

15" is pretty small for a MTB. How tall are you if you don't mind me asking? Probably a size 40 road frame but they're very difficult to find.

 

I'm 5'4", 135lbs. So yeah I'm a lightweight...
I plan to start commuting to work and just getting a feel
for what to look for. Started looking on the net and found
out bike sizes weren't interchangeable between road and
mountain.

While I'm here....

What kind of stuff should I look for in a $500-$700 used
road bike? Once a weekend warrior for mtn biking, It's been
over a year since I've really ridden routinely. So I'm not up to
date on what kind of equipment may be out there.

 

I reckon you from the USA?
$500 - 700 = ± R4700 - R6700. In SA, you can buy a new road bike for that money, something like a Giant OCR3, Schwinn Fastback, Omega Velocity, Trek 1500 etc. all with Shimano components and good wheels. Not top of the range by any means, but definitely not rubbish.
I'm not sure whats hot in terms of bikes in the US, so you'll have to do some window shopping first, make a list of what you like and what you can get for your money, come back and post the list here. From there we can tell you if it's a good buy or not in terms of the frame and the components.

 

Yes, I am from the US.  My american arrogance assumes everyone
is from the USA...

I went to take a quick look yesterday and here's my lineup
so far.

Lemond Alpe De'Huez  -- $1200 US
Trek 2200 -- $1300 US

Obviously I'm opting on a little higher price tag than I initially
intended.  I plan to use it to mostly commute, but also to do
some longer weekend rides to stay in shape.  Easy upgradeability
of the components would be nice.  I'd rather have a decent frame
and cut back to lower components.  Just some thoughts on my
brief excursion yesterday.

And back to my original post, i've found that a 52cm seems to
be a decent fit.

And also, I have some Time ATAC pedals on my mountain bike.
I'd rather not have to get different shoes (or switch cleats),
so are Time pedals interchangeable across Mountain and Road
pedals?

 

Of the two bikes you mentioned, I only know about the Trek. It's an awesome bike with great components. You want to use it for commuting and the occasional long ride? It's a real road racing machine! I know of many cyclists that would give an arm and a leg for a Trek 2200!
If you do decide on such a good bike, rather have yourself measured properly and get the right sized frame.
Regarding the pedals, I'm not sure. Maybe someone with MTB experience can shed some light?

• FRAME: Alpha™ SL Aluminum. Built from the ground up in Waterloo, Wisconsin.
• FORK: Oversized OCLV Carbon Air Rail.
• WHEELS: Bontrager Select offers aerodynamics and strength with Paired Spoke Technology™.
• COMPONENTS: Shimano Ultegra crank and 105/Ultegra 9-speed shifting. Choose a double chainring for racing or fast training rides, or triple chainring for wider gear options on longer rides.

 

There's nothing stopping you putting the same pedals on your road bike that you have on your MTB.

The Trek 2200 is a great bike.