Figuring out available memory on a device
adb shell cat /proc/meminfo
MemTotaltells you how much device memory is available outside of drivers/kernel.
MemFreetells you how much memory is currently available, checking this after booting should give you a rough guesstimate of what our max is.
To simulate what would happen in a low-memory situation you can
adb shell kill <pid> each b2g process but b2g, (Nuwa), Homescreen (use
b2g-ps to get a b2g process listing). Unfortunately Homescreen just comes back (it can be LMK'd though, so add it's USS to the MemFree total to get a better idea of an ideal MemFree value).
Get an overview of b2g application memory usage