To disable resizing completely:

textarea {
    resize: none;
}

To allow only vertical resizing:

textarea {
    resize: vertical;
}

To allow only horizontal resizing:

textarea {
    resize: horizontal;
}

Or you can limit size:

textarea {
    max-width: 100px; 
    max-height: 100px;
}

To limit size to parents width and/or height:

textarea {
    max-width: 100%; 
    max-height: 100%;
}