My approach to recovery is to separate the irreplaceable stuff from the replaceable stuff. I have my OS on C:/ and personal files, including the default Windows user folders, on D:/. If anything happens, you nuke the C:/ drive and make a fresh install, or reinstall an image, leaving your data safe. Whereas D:/ I run incremental backup programs to a HDD and use a cloud backup of some kind.
I think it depends what kind of recovery usb you mean. The Windows install files fit on 4GB sticks, but, like you say, a backup image of an installed drive might not make it onto 16GB.
Certainly if you took the Windows 10 upgrade, your activation is tied to your hardware, not a key.Unsure if you even need a key for Win10 anymore.
I'm thinking a 32Gb would be a safer bet. I installed Win7 premium on my laptop and then after all drivers were installed, tried making a backup disk/DVD and got an impending file size of 25Gb!! That shut me down quick like. I'm SURE Win10 is even bigger as well..
I think it depends what kind of recovery usb you mean. The Windows install files fit on 4GB sticks, but, like you say, a backup image of an installed drive might not make it onto 16GB.