我的设计是,

<textarea id="txt"></textarea>
<input type="button" id="btnClick" value="Submit" />

jQuery的,

$(document).ready(function () {
var field = $("#txt");
var btn = $("#btnClick");
field.on("focusin",f1);
field.on("focusout",f2);
btn.on("click",f3);

function f1() {
    field.removeClass("c1").addClass("c2");
}

function f2() {
    field.removeClass("c2").addClass("c1");
}

function f3() {
    alert('hi');
}
});

样式,

.c1 { height:40px; }
.c2 { height:250px;}

而且我还附上了Fiddle here.