Area of effect spells specify a distance, and some sort of shape as their area.

If the shape is round ( circle, sphere, cylinder) that distance is always the radius of the circle [the radius is 1/2 of the total diameter, or longest length across].

If the shape is straight or angled, then the distance is the distance for each dimension ( line, square, cube, cone). So a 20’ cube, is 20’ on each side. A 15’ cone is 15’ on each side, and 15’ connecting the ends {regarding a cone: the distance from the caster is the same as the current width of the cone – up to the max}