Hi,
I'm afraid I don't really understand the spell "Summon Elemental".
In "BRP Classic Fantasy" the spell is clarified as such:
Using the Stat-range given in BRP for the Fire Elemental it has a minimum SIZ of 9 and HP seem to based on SIZ+CON (not SIZ+CON/2)
Creating three versions Weakest - Average - Best we get:
W with 9SIZ-12HP-3POW | A with 16,5SIZ-27HP-10,5POW and B with 24SIZ-42HP-18POW
But since the stats are rolled they don't scale with SIZ. A 9SIZ (level 2) elemental might have good CON and therefore higher HP than a 12SIZ one
Furthermore since the lowest value for SIZ is 9 it doesn't seem possible to conjure one with a level 1 or 2 spell (SIZ 3 or 6).
Additionally of the most important values (Attack%, Power%, HP) only HP changes with SIZ.
HP/2 Attack% and Power% are only dependent on random characteristics.