I'm using bootstrap-sass and in that version control sizing doesn't work (e.g. form-group-sm or form-group-lg etc, see twbs/bootstrap-sass#763)

Looking into what is causing this led me to the Less version of bootstrap, and I think something is wrong with the input-size mixin and how it's being used. While the Sass version is broken, and I think the Less version works, I believe the Less version isn't generating the intended CSS.

The less code from forms.less (https://github.com/twbs/bootstrap/blob/master/less/forms.less#L315):

.input-lg,
.form-group-lg .form-control {
  .input-size(@input-height-large; @padding-large-vertical; @padding-large-horizontal; @font-size-large; @line-height-large; @input-border-radius-large);
}

Generates this CSS code:

…
textarea.input-sm,
textarea.form-group-sm .form-control,
select[multiple].input-sm,
select[multiple].form-group-sm .form-control {
  height: auto;
}
…

That doesn't look like the intended CSS. I would expect this:

…
textarea.input-sm,
.form-group-sm textarea.form-control,
select[multiple].input-sm,
.form-group-sm select[multiple].form-control {
  height: auto;
}
…

Is this wrong? I would be interested in trying to create a patch if it is.

-Caleb

This jsbin shows the bug: http://jsbin.com/nomobegivu/1/edit?html,css,output

The textarea has rows=20 set, but the output has a set height because the height: auto rule listed above isn't being matched. Remove the form-group-lg class and the text area goes back to its correct height.

I don't think the LESS fix works, but I could be mistaken. The resulting CSS I get is:
select.form-group-sm .form-control {..}
textarea.form-group-sm .form-control,
select[multiple].form-group-sm.form-control {..}

I expected:
.form-group-sm select.form-control {..}
.form-group-sm textarea.form-control,
.form-group-sm select[multiple].form-control {..}

The issue seems to be in the mixin as its pre-pending the selector. The selector is pre-pended even if the classes are nested. See this LESS example here:
http://lesscss.org/features/#parent-selectors-feature-changing-selector-order

Not sure of the fix as I played with it a bit, but by no means am I a LESS expert.

Agreed, the nonsensical selectors are still present after the "fix":
7c71b48#diff-6972cbb37f0ec48771217d4915ea7e6fR2553
CC: @mdo

This is how I got it to work without changing the mixin:

.form-group-sm .form-control 
{
    &:extend(.input-sm);
}
.form-group-sm select.form-control 
{
    &:extend(select.input-sm);
}

.form-group-sm select[multiple].form-control 
{
    &:extend(select[multiple].input-sm);
}

.form-group-sm textarea.form-control 
{
    &:extend(textarea.input-sm);
}

Do the same for lg styles. Not very elegant and a little verbose, but because of how the mixins written I don't think there are a lot of options.