Is it reasonable for smaller guns to have WAY smaller crew bases (especially if you take the ground scale into account)?
The biggest crew base I have is probably 25mm square for a German 150mm with splayed trail whereas a 105mm in towing position is probably 25mm x 15mm as the model doesn’t have the trails deployed and my 25 pdrs are probably on 20mm x 15mm bases.
Should everything be on 25mm sq bases including 2 pdrs and 25mm AT guns or can it be justified for certain guns to occupy a smaller area and thus in effect be harder to catch in a blast area?
Depends on the ground scale you’re dealing with. The gun itself, the crew themselves, the support vehicles themselves, don’t each take up much room. What takes up all the room is the mass of them together.
I wouldn’t use base area as a way of showing vulnerability of guns to CB.
Guns are amazingly hard to destroy by bombardment; easier by far to neutralise or disable the detachment, destroy the attendant vehicles and limbers, cut the telephone wires and blow up the accompanying ammo supply (WW2 was before insensitive munitions — Nebelwerfer crews were instructed to dump rockets pointing towards the enemy, in case of one being ignited by enemy action). As a rough rule of thumb, destroying a field piece requires a direct hit from a field artillery shell, or a medium shell in the gun-pit. As one of the main mechanisms of damage is putting the recuperator out of action, guns with recuperators mounted above the barrel are probably more vulnerable than those with recuperators mounted below.