I recently followed a tutorial and made my first app, and I was wondering if I could use constant value to determine rectangle sizes, etc. in my game.lau, or do I have to use the display.contentWidth, etc. to get it to scale properly?

Basically, does the resolution in the config matter? Or will everything be scaled automatically based off of that? Ex: If I put that a circle's radius = 15, will it be 15 pixels on all phones, or will it be scaled appropriately?

Things will be automatically scaled, yes.

A good way to see this would be to make a simple iPhone app with a circle (radius = 15) then view it on the iPad. You will see the difference.

Peach :)

if you set screen resolution in your config.lua
like a
application =
{
content =
{
width = 320,
height = 480,
...
}
you screen resolution always will be 320x480 on any device