Q:

How to get equal height for all sections?

I have some questions:

How can I get the nav, conversationList and conversation section expand to the bottom?
If one of these 3 expand their size (e.g. a long text). How can I apply the same size to all 3?

Fiddle Link.
HTML
<header>
    Text
</header>

<main>
    <nav>
       Navigation
    </nav>

    <section id="contentWrapper">
        <section id="contentTitle">
            TITLE
        </section>

        <section id="conversationList">
            List
        </section>

        <section id="conversation">
            Conversation
        </section>
    </section>
</main>

CSS
html, body{
    height: 100%; 
    width: 100%;
    margin:0px !important;
}

header{
    position: fixed;
    top: 0;
    height: 60px;
    width: 100%;
    background-color: #777;
    z-index: 1;
}

main{
    float: left;
    padding-top: 60px;
    background-color:black;
    bottom:0px;
    width:100%;
    min-height: calc(100% - 60px);
}

nav{
    float: left;
    width: 15%;
    min-height: 100%;
    background-color: green;
}

#contentTitle{
    float: left;
    width: 98.5%;
    height: 35px;;
    padding-top: 25px;
    padding-left: 1.5%;
    background-color: red;
}

#contentWrapper{
    float: left;
    width: 85%;
    min-height: 100%; 
}

#pageTitle{
    float: left;
    width: 100%;
    height: 40px;
    padding-top: 20px;
    padding-left: 3%;
    background-color: red;
}

#conversationList{
    float: left;
    width: 30%;
    background-color: white;
}

#conversation{
    float: left;
    width: 70%;
    background-color: yellow;
}

A:

Basic answer is to apply height: 100% to every element you want stretched. You have to remember to give each parent element a height of 100% as well, like #contentWrapper needs height: 100%.
Have a fiddle!
I have completely re-written this for you:
jsBin example!
Have a play with it so that you understand how it works.
CSS
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}
html,body {
    height: 100%;
}
header { 
    height: 60px;
    background: green;
}
main {
    display: table;
    width: 100%;
    height: calc(100% - 60px);
}
nav {
    display: table-cell;
    width: 60px;
    background: green;
}
#contentWrapper {
    display: table;
    width: 100%;
    height: calc(100% - 60px);
}
#contentTitle {
    width: 100%;
    height: 60px;
    background: red;
}
#conversationList {
    width: 80px;
    float: left;
    height: calc(100% - 60px);
    background: yellow;
}
#conversation {
    float: left;
    width: calc(100% - 80px);
    height: calc(100% - 60px);
    background: pink;
}

HTML
  <header>
    Text
</header>

<main>
    <nav>
       Navigation
    </nav>

        <section id="contentTitle">
            TITLE
        </section>

        <section id="conversationList">
            List
        </section>

        <section id="conversation">
            Conversation
        </section>

</main>

