@Carl Love 

i start if off like this: (can you please tell me what am i doing wrong and give me the right syntax?, Thank you!)

size:=proc(x0,R0,n)

local X, nextpop,initialpop,Growth;

initialpop:=x0;

Growth:=R0;

X:=Array[1..n];

for i from 1 to n do

nextpop:=(Growth*initialpop)/(1+initialpop)^4;

initialpop:=nextpop;

X[i]:=nextpop;

end do;

return X;

end proc;