Normalizing vectors to be the same size to retain PSD data

Context: PSD=periodogram(y) in MATLAB gives the PSD (power spectral density) estimate of a sound-source signal. I want to find the local maximum within 3 different x ranges of PSD.

How do I make a vector that maps 1:1 to the PSD -y value vector?The x values that MATLAB generates are Hz - frequency values, so I don't want to change the values per se, but create a vector that goes from x=0:3500, but have the same number of elements as the PSD vector, PSD= 32769*1 double.

How do I do this while retaining the same data?

PSD plot. acquired with plot(PSD) <-- x values generated by MATLAB

%% F1 range  
subset1=zeros(1, 601); %create a vector to store PSD values within the range of interest
for x=200:800;
    subset1(x)=PSD(x);% input in the values of interest
end
F1=max(subset1);% find the max value within the range
%PROBLEM: wanted to find the x value which corresponded to the max y <-- but this actually is the max y value
%PROBLEM: x vector not the same size as PSD values within the range, so many points not considered. 
disp(F1);

%% F2 range
subset2=zeros(1, 1001);
for x=800:1800;
    subset2(x)=PSD(x);
end
F2=max(subset2);
disp(F2);

%% F3 range
subset3=zeros(1, 1701);
for x=1800:3500;
    subset3(x)=PSD(x);
end
F3=max(subset3);
disp(F3);