ExpandableContentPresenter is inherited from ISContentControl, which means that it can contain a single object of any types, such as string, an image, or a panel. For more information about this content model, see Content Model Overview.

The content of ExpandableContentPresenter must be a UIElement with fixed Height and/or Width, which is used as reference for the expand and collapse animation.

Source: Intersoft ClientUI Documentation ExpandableContentPresenter.

You may try to use the ExpandableGroupBox control for your scenario. The following example shows how to define a simple UXQueryBuilder as the content of ExpandableGroupBox.

<Intersoft:ExpandableGroupBox x:Name="ExpandableContainer" VerticalAlignment="Top" >
    <Grid>
        <Intersoft:UXQueryBuilder x:Name="QueryBuilder"/>
    </Grid>
</Intersoft:ExpandableGroupBox>

It is best practice to define the size in the content element of the ExpandableGroupBox instead of defining it in the control itself. This allows the ExpandableGroupBox to support fluid layout and perform smoother operation.

Hope this helps.