Try it out, post your CSS it will make it easier to follow, this is what I think you have:
#wrapper {
height: 600px;
width: 600px;
border: 2px solid black;
}
#textbox {
height: 25%;
border: 2px solid red;
}
#send {
height: 100%;
border: 2px solid blue;
}
Which put into html would look like...