I need to make a 7 3" sculpture to a way smaller scale. How big should it be?

It depends on the purpose: if it is to sown the model's context with the surrounding landscape then it is a different matter to showing the way the inscriptions align on the memorial itself. So anywhere from 1:500. Action figures are in the range of 1 to 6: that will be quite a model at that scale and it will look crude unless you place a lot of detail into it. Model railway people are commonly rounded off to 1:100 or 1:200. That is a more comfortable scale but you would need to calculate to see if that is still manageable. So sorry for the vague answer- it is really 'horses for courses' so I can't appreciate what you are after.

A link to images of the figures would have been nice, but the numbers you give suggest you want to scale down some or all of the 19 stainless steel figures https://en.wikipedia.org/wiki/Korean_War_Veterans_... and not the whole memorial.

As this http://actionfigures.about.com/od/gettingstarted/a... says, there are many common action figure scales but the two you should probably consider are 1:12 and 1:6 and do that on the basis of how big your figures are to be: If the figures are to end at about 6" tall, then 1:12 to real people is correct, while if they are to be about 12" tall, the 1:6 is correct.

But you are scaling down from over-sized figures so you have to pick some relationship. I would suggest figuring the 7'6" tallest figures are 6'0" tall people with combat gear (boots and helmets) adding to height, so you are scaling 7'6" (90") down to 6'3" (75") size. For all the work you will be doing, I would think 12" model figures would be better instead of the small 6" action figure size. which leads to 0.1388889 or 0.14.

At this scale the 7'6" statue would be 12.6" tall.

The easiest way to scale would probably be to print photographs of the statues scaled to size and measure off of them. To get the measurements/print outs scaled you would need accurate measurements of each of the statues which you can get when you take lots of photos of the originals that you will need for modeling.