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 defined(HAVE_CONFIG_H)
#include <config/bitcoin-config.h>
#endif
#include <qt/optionsmodel.h>
#include <qt/bitcoinunits.h>
#include <qt/guiutil.h>
#include <interfaces/node.h>
#include <validation.h> 
#include <net.h>
#include <netbase.h>
#include <txdb.h> 
#include <qt/intro.h>
#include <QNetworkProxy>
#include <QSettings>
#include <QStringList>
const char *DEFAULT_GUI_PROXY_HOST = "127.0.0.1";
static const QString GetDefaultProxyAddress();
OptionsModel::OptionsModel(interfaces::Node& node, QObject *parent, bool resetSettings) :