Android is a mobile operating system developed by Google, based on a modified version of the Linux kernel and other open source software and designed primarily for touchscreen mobile devices such as smartphones and tablets.
Herein, what is a custom OS for Samsung?
“ROM” stands for “read-only memory.” A custom ROM replaces your device's Android operating system — normally stored in read-only memory — with a new version of the Android operating system. Custom ROMs are different from acquiring root access.
How long does it take to install a custom OS?
I flashed MIUI 6 in my old HTC Desire 616 and it took approx 10 to 15 minutes to flash and boot, First boot after flashing a room usually takes time so keep patience. But if want to know that how much time it takes to download a ROM from the Internet then It depends on Your Internet Connection speed.
What does OS mean for size?
OS just means "one size" as in one size fits most. These fit adults and older kids just fine.