With a pruned wallet possible in master I believe we should expose the option in the GUI.

The Main tab in preferences will offer a chance to toggle the node type and size.
<img width="612" alt="pruning_gui_off" src="https://cloud.githubusercontent.com/assets/216917/8766147/834fb842-2e2c-11e5-94b0-c342132243bf.png">
<img width="612" alt="pruning_gui_on" src="https://cloud.githubusercontent.com/assets/216917/8766148/8351a0c6-2e2c-11e5-841b-af02fd77ff44.png">

The prune size UI should not permit sizes less than 550.

{
    ui->okButton->setEnabled(fState);
}
void OptionsDialog::on_resetButton_clicked()
{
    if(model)
    {
        
        QMessageBox::StandardButton btnRetVal = QMessageBox::question(this, tr("Confirm options reset"),
            tr("Client restart required to activate changes.") + "<br><br>" + tr("Client will be shut down. Do you want to proceed?"),
            QMessageBox::Yes | QMessageBox::Cancel, QMessageBox::Cancel);
        if(btnRetVal == QMessageBox::Cancel)
            return;
        
        model->Reset();
        QApplication::quit();
    }
}
void OptionsDialog::on_openBitcoinConfButton_clicked()