With custom paging component: the page size is always 20 even autopaging is setting to true.
<zk>
<zscript><![CDATA[
Integer[] items = new Integer[30];
for (int i = 0; i < 30; i++)
items[i] = i;
]]></zscript>
<window title="new page title" border="normal" width="100%" vflex="1">
<div vflex="true">
<paging id="paging" />
<grid id="listbox" mold="paging" paginal="${paging}" vflex="true" autopaging="true">
<columns>
<column label="column1" />
</columns>
<rows>
<row forEach="${items}">${each}</row>
</rows>
</grid>
<button label="test" onClick="alert(listbox.getPaginal().getPageSize())" />
</div>
</window>
</zk>

Without other paging component, the page size will change according to browser height size.
<zk>
<zscript><![CDATA[
Integer[] items = new Integer[30];
for (int i = 0; i < 30; i++)
items[i] = i;
]]></zscript>
<window title="new page title" border="normal" width="100%" vflex="1">
<div vflex="true">
<grid id="listbox" mold="paging" vflex="true" autopaging="true">
<columns>
<column label="column1" />
</columns>
<rows>
<row forEach="${items}">${each}</row>
</rows>
</grid>
<button label="test" onClick="alert(listbox.getPaginal().getPageSize())" />
</div>
</window>
</zk>