Resize Prim, 1 direction

I hope this is a quicky, I want to increase/decrease a prim's size at one end (not both). The below script changes the z axis as the sun angle changes but does it to both ends (think thermometer here). Is the a way to direct the change to the upper part of the prim or do I have to reposition the whole prim each time? Many thanks as always.

//Code//

default
{
state_entry()
{
\\llSay(0, "Hello, Avatar!";
}

touch_start(integer total_number)
{
vector sun = llGetSunDirection();
if(sun.z > 0)

{
float FLTz = sun.z;
llSay(0, "Touched.";
llSetPrimitiveParams([PRIM_FULLBRIGHT,ALL_SIDES,FALSE]);
llSetPrimitiveParams([PRIM_COLOR, ALL_SIDES, <1.0,0.0,0.0>, 0.7]);
llSetPrimitiveParams([PRIM_SIZE, <0.20,0.20,FLTz>]);
}
}
}

//Code Ends//

yes, you have to move the prim too by half of what it was stretched

llSetPrimitiveParams([PRIM_POSITION,llGetPos() + <0.0,0.0,FLTz/2>])

you may also want to do all the llSetPrimitiveParams calls in one, because it gives a .2 second delay for each one so the next one won't get done until after the delay

llSetPrimitiveParams([PRIM_FULLBRIGHT,ALL_SIDES,FALSE,
PRIM_COLOR, ALL_SIDES, <1.0,0.0,0.0>, 0.7,
PRIM_SIZE, <0.20,0.20,FLTz>,
PRIM_POSITION,llGetPos() + <0.0,0.0,FLTz/2>]);

edit:
just in case you have the prim rotated other than zero, this will make it move along it's own z axis, instead of the world's axis:

PRIM_POSITION,(llGetPos() + <0.0,0.0,FLTz/2>*llGetRot()

I had to do something similar on a voting tool to show a dynamic bar graph (the SLOODLE Choice tool). If you path cut a prim in half (e.g. start the cut at 0.25 and end it at 0.75) then scaling it in script will only appear to resize it in one direction. That way you can avoid the movement.

You guys are the best, I'll try both suggetions tonight.

problem with that is you can only scale it up to 5m. of course the op's script looks like it's only going up to 1 meter anyways. and i just noticed my example still moves it up even if the scale gets smaller. so the movement would need more logic. maybe a start position and then when it scales it would be PRIM_POSITION, (startpos + <0.0,0.0,FLTz/2>*llGetRot()