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.

break;
        case MinimizeOnClose:
            fMinimizeOnClose = value.toBool();
            settings.setValue("fMinimizeOnClose", fMinimizeOnClose);
            break;
        
        case ProxyUse:
            if (settings.value("fUseProxy") != value) {
                settings.setValue("fUseProxy", value.toBool());
                setRestartRequired(true);
            }
            break;
        case ProxyIP: {
            auto ip_port = GetProxySetting(settings, "addrProxy");
            if (!ip_port.is_set || ip_port.ip != value.toString()) {
                ip_port.ip = value.toString();
                SetProxySetting(settings, "addrProxy", ip_port);
                setRestartRequired(true);
            }
        }
        break;
        case ProxyPort: {
            auto ip_port = GetProxySetting(settings, "addrProxy");
            if (!ip_port.is_set || ip_port.port != value.toString()) {
                ip_port.port = value.toString();
                SetProxySetting(settings, "addrProxy", ip_port);
                setRestartRequired(true);
            }
        }
        break;