Input size vs width

size is inconsistent across different browsers and their possible font settings.

The width style set in px will at least be consistent, modulo box-sizing issues. You might also want to set the style in ‘em’ if you want to size it relative to the font (though again, this will be inconsistent unless you set the input's font family and size explicitly), or ‘%’ if you are making a liquid-layout form. Either way, a stylesheet is probably preferable to the inline style attribute.

You still need size for <select multiple> to get the height to line up with the options properly. But I'd not use it on an <input>.

I want to say this goes against the "conventional wisdom", but I generally prefer to use size. The reason for this is precisely the reason that many people say not to: the width of the field will vary from browser to browser, depending on font size. Specifically, it will always be large enough to display the specified number of characters, regardless of browser settings.

For example, if I have a date field, I typically want the field wide enough to display either 8 or 10 characters (two digit month and day and either two or four digit year, with separators). Setting the size attribute essentially guarantees me that the entire date will be visible, with minimal wasted space. Similarly for most numbers - I know the range of values expected, so I'll set the size attribute to the proper number of digits, plus decimal point if applicable.

As far as I can tell, no CSS attribute does this. Setting a width in em, for example, is based off the height, not the width, and thus is not very precise if you want to display a known number of characters.

Of course, this logic doesn't always apply - a name entry field, for example, could contain any number of characters. In those cases I'll fall back to CSS width properties, typically in px. However, I would say the majority of fields I make have some sort of known content, and by specifying the size attribute I can make sure that most of the content, in most cases, is displayed without clipping.

I suggest, probably best way is to set style's width in em unit :) So for input size of 20 characters just set style='width:20em' :)

You can use both. The css style will override the size attribute in browsers that support CSS and make the field the correct width, and for those that don't, it will fall back to the specified number of characters.

Edit: I should have mentioned that the size attribute isn't a precise method of sizing: according to the HTML specification, it should refer to the number of characters of the current font the input will be able to display at once.

However, unless the font specified is a fixed-width/monospace font, this is not a guarantee that the specified number of characters will actually be visible; in most fonts, different characters will be different widths. This question has some good answers relating to this issue.

The snippet below demonstrates both approaches.

@font-face {
    font-family: 'Diplomata';
    font-style: normal;
    font-weight: 400;
    src: local('Diplomata'), local('Diplomata-Regular'), url(https://fonts.gstatic.com/s/diplomata/v8/8UgOK_RUxkBbV-q561I6kFtXRa8TVwTICgirnJhmVJw.woff2) format('woff2');
    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02C6, U+02DA, U+02DC, U+2000-206F, U+2074, U+20AC, U+2212, U+2215;
}
@font-face {
    font-family: 'Open Sans Condensed';
    font-style: normal;
    font-weight: 300;
    src: local('Open Sans Condensed Light'), local('OpenSansCondensed-Light'), url(https://fonts.gstatic.com/s/opensanscondensed/v11/gk5FxslNkTTHtojXrkp-xBEur64QvLD-0IbiAdTUNXE.woff2) format('woff2');
    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02C6, U+02DA, U+02DC, U+2000-206F, U+2074, U+20AC, U+2212, U+2215;
}
p {
  margin: 0 0 10px 0;
}
input {
  font-size: 20px;
}
.narrow-font {
  font-family: 'Open Sans Condensed', sans-serif;
}
.wide-font {
  font-family: 'Diplomata', cursive;
}
.set-width {
  width: 220px;
}

<p>
  <input type="text" size="10" class="narrow-font" value="0123456789" />
</p>
<p>
  <input type="text" size="10" class="wide-font" value="0123456789" />
</p>
<p>
  <input type="text" size="10" class="narrow-font set-width" value="0123456789" />
</p>
<p>
  <input type="text" size="10" class="wide-font set-width" value="0123456789" />
</p>