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.

#if QT_VERSION >= 0x040800
            
            ui->lang->addItem(locale.nativeLanguageName() + QString(" (") + langStr + QString(")"), QVariant(langStr));
#else
            
            ui->lang->addItem(QLocale::languageToString(locale.language()) + QString(" (") + langStr + QString(")"), QVariant(langStr));
#endif
        }
    }
#if QT_VERSION >= 0x040700
    ui->thirdPartyTxUrls->setPlaceholderText("https://example.com/tx/%s");
#endif
    ui->unit->setModel(new BitcoinUnits(this));
    
    mapper = new QDataWidgetMapper(this);
    mapper->setSubmitPolicy(QDataWidgetMapper::ManualSubmit);
    mapper->setOrientation(Qt::Vertical);
    
    ui->proxyIp->setCheckValidator(new ProxyAddressValidator(parent));
    ui->proxyIpTor->setCheckValidator(new ProxyAddressValidator(parent));
    connect(ui->proxyIp, SIGNAL(validationDidChange(QValidatedLineEdit *)), this, SLOT(updateProxyValidationState()));
    connect(ui->proxyIpTor, SIGNAL(validationDidChange(QValidatedLineEdit *)), this, SLOT(updateProxyValidationState()));
    connect(ui->proxyPort, SIGNAL(textChanged(const QString&)), this, SLOT(updateProxyValidationState()));
    connect(ui->proxyPortTor, SIGNAL(textChanged(const QString&)), this, SLOT(updateProxyValidationState()));
}
OptionsDialog::~OptionsDialog()
{
    delete ui;
}
void OptionsDialog::setModel(OptionsModel *_model)