You need at least 1GB unless you have removed sysresccd-installer-1.1.2.exe from the official version. Your BIOS must be able to boot from USB hard disks, and the USB device must be defined before other devices in the boot devices order. The other methods are more complicated and are more likely to fail.
You may also be interested in making a backing store to keep your changes between reboots. This will modify the iso in place. You just need an empty directory on your system on which the ISO image can be mounted. Plug in the USB stick Just make sure the USB stick has been plugged in, and wait a few seconds to be sure the device has been detected by the system. Execute the installer Now you just have to execute the installation script which is at the root of the CD-ROM. This script requires several commands to run but it won’t be a problem.
This script will show you a list of USB sticks detected on your system. Only removable medias are in the list. This way it’s not possible to destroy a persistent device by accident. It can be executed directly as it requires no installation on the hard disk. If you have a Linux system on your computer you can also use the first method, which does not require booting from a CD-ROM at all. Formatting the USB stick will remove all of its contents, so make sure you don’t need that contents, or make a backup first. Boot from that CD-ROM with the default boot options.