Was able to figure this out. Just created a
PopupStyle="{StaticResource RadListPickerPopupStyleWrap}" in my RadListPicker and put this in the page resources:
<Style x:Key="RadListPickerPopupStyleWrap" TargetType="telerikPrimitives:RadDataBoundListBox">
<Setter Property="FontSize" Value="{StaticResource PhoneFontSizeExtraLarge}"/>
<Setter Property="FontFamily" Value="{StaticResource PhoneFontFamilySemiLight}"/>
<Setter Property="Background" Value="{StaticResource PhoneChromeBrush}"/>
<Setter Property="HorizontalAlignment" Value="Stretch"/>
<Setter Property="VerticalAlignment" Value="Stretch"/>
<Setter Property="VirtualizationStrategyDefinition">
<Setter.Value>
<telerikPrimitives:WrapVirtualizationStrategyDefinition Orientation="Horizontal" WrapLineAlignment="Center"/>
</Setter.Value>
</Setter>
<Setter Property="CheckBoxStyle">
<Setter.Value>
<Style TargetType="telerikPrimitives:ItemCheckBox">
<Setter Property="VerticalAlignment" Value="Center"/>
<Setter Property="Margin" Value="0, 0, -18, 0"/>
</Style>
</Setter.Value>
</Setter>
</Style>