You pick "A"a point", and affect everyone "within 5 feet". a A point is usually an intersection, and 5 foot is 1 square. This means that 'call lighning'call lightning has a 2*2 square2-square-by-2-square area.
Arms of Hadar, with a 10 foot-foot (not square10-square) radius, affects everyone within 2 squares, so a 5*5 square5-square-by-5-square area, centered on the caster (a potential maximum of 24 targets {25 total, minus one for you}).