The maximum amount of memory that your app can use will vary depending on the device. On this page, you can see the amount of RAM that various Apple devices have:
http://en.wikipedia.org/wiki/List_of_iOS_devices
Your app can use approximately 50% of the RAM before iOS kills the app.
If you use the default settings for CoronaSDK, your app can run on iOS 6 devices. Your app will not run on anything that has “Highest supported operating system” older then iOS 6.0.
So, it looks like 256 MB is the smallest amount on any supported device. Staying under 50% of that should be OK for iOS, If you’re planning to port your app to Android, I would recommend trying to stay well below that number.