How much storage space is required to update to iOS 12 wirelessly?

Download is just under 1.5GB. If you are worried about not having sufficient storage space best to do the update connected to a computer as the content of the device is then stored on to the computer temporarily to enable the update to take place.

You definitely need more space on the device than the actual download as the original OS will remain on the device until the new one is safely installed.

The size of the download itself is 1.36 GB on my iPhone 5s.

Just guessing, but you likely need anywhere from 5 - 6gb free space on your phone.

But typically, you need more space than the actual size of the download to do it over air successfully.

Not me. Every iOS update that I have done since iOS 5 came out was done OTA. I have also done every update while I was at work. I’m getting old now and not as confident as I used to be, so I am waiting until I get home and I will update there with my MBP close by my side, just in case.

I don't know why I thought you had an aversion to OTA updates. Like you, all mine have been OTA since OTA became available. I've never had an issue yet. I do them at work too. I've already updated my phone and watch. I get to go home and update another phone, two iPads and another watch this evening!

I’m just stating what the size of the download is, that’s all. I don’t remember how it works when the software actually installs on the device.

If I remember correctly, you don't trust over the air updates. Is my memory still good?