In ruby:

def groups(from, to, max)
   size = to - from + 1
   undivisable = (size % max).to_f
   group_step = (size.to_f / max).floor + (undivisable / max).floor
   leftovers = undivisable % max

   result = []
   last_num = 0
   max.times do
     next_num = last_num + group_step
     if leftovers > 0
       next_num += 1
       leftovers -= 1
     end
     range = (last_num + 1)..(next_num)
     last_num = next_num
     result.push range
   end
   result
end

puts groups(1, 7, 5).inspect
puts groups(1, 9, 5).inspect
puts groups(1, 19, 5).inspect

[1..2, 3..4, 5..5, 6..6, 7..7]
[1..2, 3..4, 5..6, 7..8, 9..9]
[1..4, 5..8, 9..12, 13..16, 17..19]