2 things

Local spa dealer tries to upsell from 25 sq foot filter - is that worth doing?

secondly, I have an older nordic hot tub - noticed slight leak on outlet side of heater. It has a "split" type of union that screws together. unable to stop leak. I tightened it - got better but won't seal. Is this a standard oring size or best to pull it & match it. It has (I believe) a gecko powerpack / heater assy. I hve loosened this union only once since purchase when I replaced the pressure / flow switch

What is he trying to sell you? I would go much larger on the square footage or you may have a "flo" issue where the pump is not pushing enough water through the heater and a flo error code occurs.
Most heater unions will have an oring but some have what they call a ridge oring where it is flat on one side and has a raised ridge on the other side. Most will be either 1.5" union or 2" unions.