The iPhone Mail app has no HTML signature field. Email the signature to yourself from a desktop app, paste it into Settings, then shake the phone to restore the sizing.
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Install it on a computer first
Install the signature in a desktop app first, then send yourself an email so the message contains it. Apple Mail, Outlook for Mac, new Outlook or Thunderbird all work.
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Copy the signature on your iPhone
Open that email in the Mail app. Press and hold a plain piece of the signature text (not a link), tap Select All, then Copy.
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Paste into Mail settings
Open Settings → Apps → Mail → Signature. On older iOS this is Settings → Mail → Signature. Choose All Accounts or Per Account, then press and hold to Paste.
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Shake to fix the size
The signature will look too big at first. Shake the phone. When Undo Change Attributes appears, tap Undo.
If shaking does nothing, turn on Settings → Accessibility → Touch → Shake to Undo. You can also swipe left with three fingers, or tap Undo on the keyboard if it is showing. Then create a new email to check it.