(Assuming this is the Windows Phone Toolkit ListPicker)
If your list is longer than 5 items then it will open in FullScreenMode. It isn't possible to change this threshold value.
So, this one will expand:
<toolkit:ListPicker Header="Background">
<sys:String>dark</sys:String>
<sys:String>light</sys:String>
<sys:String>dazzle</sys:String>
<sys:String>4</sys:String>
<sys:String>5</sys:String>
</toolkit:ListPicker>
This one will always be full screen:
<toolkit:ListPicker Header="Background">
<sys:String>dark</sys:String>
<sys:String>light</sys:String>
<sys:String>dazzle</sys:String>
<sys:String>4</sys:String>
<sys:String>5</sys:String>
<sys:String>6</sys:String>
</toolkit:ListPicker>