As far as I'm aware, stack sizes for blocks are hard coded, so they can't be edited by xml means. It would involve editing the .dll, I believe, which I have no knowledge in.
However, I can suggest using wood logs for your fuel, as it has a full five minutes of burn time. Or you can modify the fuelvalue of the wood frame by editing the woodframemaster block...
<block id="149" name="woodFrameMaster">
<property name="Group" value="Building" />
<property name="DescriptionKey" value="woodFrameGroupDesc" />
<property name="Material" value="wood_weak" />
<property name="Shape" value="New" />
<property name="LightOpacity" value="0" />
<property name="Model" value="cube" />
<property name="Mesh" value="cutout" />
<property name="Frame" />
<property name="Texture" value="381" />
<property name="ImposterExclude" value="true" />
<property name="ImposterDontBlock" value="true" />
<property name="CreativeMode" value="None" />
<property name="Collide" value="movement,melee,rocket" />
<!-- -rocket -->
<property name="FuelValue" value="150" />
<property class="RepairItems">
<property name="wood" value="2" />
</property>
<!-- <property name="CustomPlaceSound" value="place_block_wood"/> this works but was too annoying -->
<property class="UpgradeBlock">
<property name="ToBlock" value="woodMaster" />
<property name="Item" value="wood" />
<property name="ItemCount" value="4" />
<property name="UpgradeHitCount" value="4" />
</property>
<property name="CustomUpgradeSound" value="place_block_wood" />
<drop event="Destroy" name="wood" count="1" prob="1" />
<drop event="Fall" name="woodDebris" count="1" prob="0.3" stick_chance="1" />
<property name="EconomicValue" value="5" />
<property name="EconomicBundleSize" value="20" />
</block>