What a wide signature does to a phone
An email's text wraps to fit whatever screen it is read on. A signature built from tables does not wrap: it is as wide as it was drawn. On a desktop that never matters, because a reading pane is several times wider than any signature. On a phone it decides how the whole email looks.
When we tried wide signatures on handsets, Apple Mail on an iPhone shrank the entire message until the widest thing in it fitted the screen, so the sender's signature made the recipient's text smaller. Gmail and Outlook on phones kept the text at its size and let the signature run off the edge, to be scrolled sideways. Neither is a reader's fault, and both make an email look broken.
The time we chose 480
The first standard size we settled on was 480 pixels wide. It looked comfortable in the builder, which runs on a laptop, and nobody tried it on a phone before it was everywhere. When we did, it was too wide for every handset we held, including the largest iPhone, and every design had to be redrawn.
The lesson was not about 480. It was that a signature's size has to be decided on the smallest screen it will land on, because that is the only place it can go wrong.
Why 320
The narrowest phone a signature is likely to meet lays out at 320 pixels. The first iPhone SE's screen is 640 pixels across at twice the usual density, so an email on it is laid out 320 wide, and phones since then are wider. A design that is 320 wide fits every one of them without shrinking anything.
It is a floor, not a comfortable middle: 320 fills a 320 screen exactly, so a mail app that adds a margin of its own pushes it over by that margin. We chose to accept that on the smallest phones rather than make every signature narrower everywhere.
The banner under a signature follows the same rule. It is always 320 pixels wide, whatever the design above it, and an uploaded banner is framed to 320 by 80 before it is used.
Pinned, not described
Writing "320" in a design's settings does not make it 320. A table cell's width is only a suggestion to a mail client, and a table with nothing holding it shrinks to its contents, so every design carries the width and the height on its outer table and on the card inside it, and a test fails a design that only declares its size.
Even that has to be measured where the signature lands, not where it is made. Not long ago three of our designs were 160 pixels tall in the builder and 184 in every inbox: the builder counts a cell's padding inside its height and a mail client adds it on top. They are 160 everywhere now, and a test fails any design that pads a cell it gives a height.
The other thing that widens a card is a long word, and in a signature the longest word is an email address, which has nowhere to break. When we measured every design with long addresses on 27 October 2026, six of them grew past 320, one to 371 pixels with a 28-character address. Now a long address breaks onto another line in every design instead. That costs a line of height, which a phone scrolls past, rather than width, which makes it shrink the whole email.
Small, and still sharp
A signature drawn at 320 pixels is shown on phones whose screens pack two or three pixels into each of those. Text stays sharp on its own; pictures do not. So every photo is rendered at twice the size it is drawn, and the mail client scales it down, which keeps the face crisp on a dense screen and smooths the edge of a cut-out shape.
The one design that is not 320 by 160 is the portrait one, which stands taller and narrower. It is the exception on purpose, and it is narrower, not wider, for the same reason as everything else here.