Bootstrap 3 .input-xs, já que ele não existe.

.input-xs {
    height: 22px;
    padding: 5px 5px;
    font-size: 12px;
    line-height: 1.5;
    border-radius: 3px;
}

Uso de amostra em input-group

<div class="input-group">
    <span class="input-group-btn">
        <button class="btn btn-xs btn-success">
            <i class="glyphicon glyphicon-plus"></i>
        </button>
    </span>
    <input type="text" class="form-control input-xs" />
    <span class="input-group-btn">
        <button class="btn btn-xs btn-danger">
            <i class="glyphicon glyphicon-minus"></i>
        </button>
    </span>
</div>

Alternativamente, você pode adicionar uma classe a input-groupesta:

.input-group-xs>.form-control,
.input-group-xs>.input-group-addon,
.input-group-xs>.input-group-btn>.btn {
    height: 22px;
    padding: 1px 5px;
    font-size: 12px;
    line-height: 1.5;
}

<div class="input-group input-group-xs">
    <span class="input-group-btn">
        <button class="btn btn-success">
            <i class="glyphicon glyphicon-plus"></i>
        </button>
    </span>
    <input type="text" class="form-control" />
    <span class="input-group-btn">
        <button class="btn btn-danger">
            <i class="glyphicon glyphicon-minus"></i>
        </button>
    </span>
</div>

O que for melhor para você