How can I solve this loop?

I have the following:

R=[-0.1 0 0.1 0.2 0.3 0.4]

Length=20;
theta=(P_hight-Diam)/Length;

for i=1:1:5;
    if R(i)>0 & R(i)>theta;
       M(i)=1;
    end
end

a1=sum(M);

When I make my Length bigger a1 also increase. I would like to make a loop which give me the minimum Length where a is the bigger.

Can some one help me?

[Wayne King]

Can you please clarify what you want. Increasing Length will reduce the value of theta and therefore I'm not surprised that M would have more nonzero entries and the sum would be larger.