Hi , the size of the disc is generated by the focal length of the scope ( eg magnification ) when using any given eyepiece just like any telescope .
I had ( and loved ) a Coranado 'Solarmax 40mm and it would show both surface detail and prominances very well just by tuning the elation , PST's cant do that but I am sure Lunts have a similar system , that's all you need here .
On the blocking filter size , that is only a mask in the diagonal that limits the FOV , not magnification .
Hope this helps .
One more thing ,, aint Ha solar observing addictive ??
, I should never have sold mine .
Brian.