[amibroker] How to enter a different position size if short

[cpescho]

What is the easiest way to tell Amibroker to enter a short position with lets 
say half the position size of a long position or vice versa?

I tried this but it didnt work

PositionSize = 5000; 
Buy = Cross( Signal(), MACD() );
Sell = Cross( MACD(), Signal() );
Short = Cross( MACD(), Signal() );
Cover  = Cross( Signal(), MACD() );
SetPositionSize(50, IIf(Short, spsPercentOfPosition,spsNoChange));