The speed is determined by the length of the vector. So you can either normalize this length and multiply with 250:
onready var velo = set_linear_velocity(Vector2(rand_range(250, -250),rand_range(250, -250)).normalized() * 250)
Or you can simply use a randomly rotated Vector with length 250:
onready var velo = set_linear_velocity(Vector2(250,0).rotated(randf() * 2.0 * PI))