Hi, according to the documentation, in scatterm(lat,lon,s,c) the area of each marker is determined by the magnitudes in the vector s.
Still if you are not able to use different size markers then use the code below, which travers through the vector sand plots different sizes of the marker based upon the given condition.
worldmap([-49 -47.5],[-150 -147.5])
if z(i)> (-850)
Hope this helps!