Why do some emails look tiny when I read them on my phone?

Often because something in them is wider than the screen, such as a wide signature or banner, and Apple Mail shrinks the whole email to fit it. A signature no wider than 320 pixels never causes that.

Will people see my signature's animation on their phones?

Gmail, Apple Mail and Outlook on phones play animated GIFs. The few readers whose client shows only the first frame see the finished photo, because every animation here starts on it.

Mobile Email Signatures: Reading and Sending on a Phone

When your email is read on a phone

Many of the people you write to will read your email on a phone, so the phone is where a signature has to work. Three things decide whether it does.

Width. A signature wider than the screen makes Apple Mail shrink the whole email to fit, and other apps let it run off the edge. The narrowest phone a signature is likely to meet lays out at 320 pixels, so a signature 320 wide fits them all. Every size, explained.
Sharpness. Phone screens pack two or three pixels into each pixel of an email, so pictures made at exactly their shown size look soft. Make them at twice that.
Taps. A phone number linked with tel dials when it is tapped, so the number in a signature is one tap from a call.

Phones are also where dark mode is most often on, which can change a signature's colours.Signatures in dark mode.

When you write on a phone

Writing is the harder direction. The Gmail app and the Outlook app each have a signature setting, and each stores plain text only. Paste a signature into one and it keeps the words and throws away the photo, the logo, the colours and the links. There is no setting that changes this.

iPhone and iPad: the exception

iPhone Mail is the one mobile app that keeps formatting, but only if the signature arrives by email.

Email the signature to yourself. On a computer, install the signature in a desktop mail app first, then send yourself any message. That message now carries the signature.
Open it on your iPhone and copy it. Open that email in the Mail app. Press and hold on the signature, choose Select All, then Copy. Copying it from a real message is what preserves the images.
Paste it into the signature setting. Go to Settings, then Apps, then Mail, then Signature. Clear whatever is there and paste. The formatting comes through.
Check it landed. Start a new message. If the signature came through as plain text, the copy step picked up text only. Go back and use Select All rather than dragging a selection. If you send from more than one address, choose Per Account rather than All Accounts.

Writing from the web on your phone

The other way round the plain-text limit is to skip the app. Open your mail's web version in the phone's browser, Gmail or Outlook on the web, and write from there: it uses the signature you installed on a computer, formatting and all. It is less convenient than the app, so it suits the emails where the signature matters, such as a first message to a new client.

A short signature for the phone

For everything else, give the phone app a short plain-text signature on purpose rather than leaving its default: your name, your role and company, and the one number people should call. Replace "Sent from my iPhone" with it, so a reply from your phone still says who it is from and how to reach you.

Email signatures on phones