Firstly apologies as I think I have gone a bit over the top with this forum post!
Anyways, I have gone back into Wix Editor myself to check this and yes, whereas you can change the text down to 6pt on the desktop editor, when you come to change it on the mobile version, even though you can click the button down to 6pt and it greys out, the actual visual size of the text won’t go beyond a set size.
I think this isn’t a bug within Wix, I think it done for a reason as all the editing of the text is done through the Desktop Editor and then Wix optimise it for the Mobile Editor.
You can change a few text settings on it like the size, however I think it is done intentionally so that you can’t make it too small for practicality.
The recommended text size for reading easily is 9-10pt with a leading of 12-14pt (leading is the gap between lines which Wix class as line spacing.)
Anything under 7pt in text size is too small in reality and you have to think that you need your website to be viewable by everybody and not just people with perfect eyesight who can read the fine smallprint.
You shouldn’t need to go too small with the text size on your mobile phone, remembering that everybody’s phones screen size are different, so if you make it small to begin with, then some phones display will make it even smaller when it is shrunk to fit their own screen display.
If you setup the word Email in your user input as the placeholder text, then it will disappear when the user clicks on the box to type in their email anyway.
Plus, note that emails are all different sizes anyway too, so don’t try to find a font size that you think would accommodate the most sizes of email addresses and be able to for it all to be shown in the user input at one time without having to scroll across.
Again, apologies if this got a bit long winded!