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->tabWidget->removeTab(ui->tabWidget->indexOf(ui->tabWindow));
#endif
    
    if (!enableWallet) {
        ui->tabWidget->removeTab(ui->tabWidget->indexOf(ui->tabWallet));
    }
    
    QDir translations(":translations");
    ui->bitcoinAtStartup->setToolTip(ui->bitcoinAtStartup->toolTip().arg(tr(PACKAGE_NAME)));
    ui->bitcoinAtStartup->setText(ui->bitcoinAtStartup->text().arg(tr(PACKAGE_NAME)));
    ui->openBitcoinConfButton->setToolTip(ui->openBitcoinConfButton->toolTip().arg(tr(PACKAGE_NAME)));
    ui->lang->setToolTip(ui->lang->toolTip().arg(tr(PACKAGE_NAME)));
    ui->lang->addItem(QString("(") + tr("default") + QString(")"), QVariant(""));
    for (const QString &langStr : translations.entryList())
    {
        QLocale locale(langStr);
        
        if(langStr.contains("_"))
        {
#if QT_VERSION >= 0x040800
            
            ui->lang->addItem(locale.nativeLanguageName() + QString(" - ") + locale.nativeCountryName() + QString(" (") + langStr + QString(")"), QVariant(langStr));
#else
            
            ui->lang->addItem(QLocale::languageToString(locale.language()) + QString(" - ") + QLocale::countryToString(locale.country()) + QString(" (") + langStr + QString(")"), QVariant(langStr));
#endif
        }
        else
        {