Mob size & age

I was trying to make a custom ghast & found out they spawn smaller then a normal ghast.  This size turned out really good for a flying eyeball, but it is too small for new ghast types or new slime types.   When I pick the ghast or slime as the base type for my custom mob, I can't see how to make them spawn with the right size setting.

My slime is at the smallest setting, so I named him mini slime & made it spawn in swamps because it will only spawn in that small size.

I would like to make a ghast with a custom skin & have it spawn as the normal size of a ghast

I would like my slime to spawn in a larger size so it can split into 4 smaller slime mobs rather then just die

if there is a way to do this, to set the size a mob will spawn naturally, then can we also set the age?  To get custom skin villagers of both adult & baby.

Yet I don't see how to set age or size. Do I need a custom mob model or is there a way to set the size for ghast/slime or the age for cow/pig/villager