The size of the indicated variable or array appears to be changing with each loop iteration. Commonly, this message appears because an array is growing by assignment or concat

clc
clear

h = 1;  
r = 0:h:20; 
z = 0:h:20; 

[rgrid,zgrid] = meshgrid(r,z);
zgrid = flip(zgrid);

b1=[]; 
for i=1:size(rgrid,1)*size(zgrid,2)
    if zgrid(i) == min(min(zgrid))
       b1 = [b1 i];
    end
end

[Stephen23]

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

Answer 1

[Stephen23]

h = 1;  
r = 0:h:20; 
z = 0:h:20; 
[rgrid,zgrid] = meshgrid(r,z);
zgrid = flip(zgrid)

zgrid = 21×21

20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11

Simpler, faster code:

b2 = find(zgrid==min(zgrid(:))).'

b2 = 1×21

21 42 63 84 105 126 147 168 189 210 231 252 273 294 315 336 357 378 399 420 441

or even simply:

n3 = numel(z);
b3 = n3:n3:n3*numel(r)

b3 = 1×21

21 42 63 84 105 126 147 168 189 210 231 252 273 294 315 336 357 378 399 420 441

Your code:

b1 = []; 
for i = 1:size(rgrid,1)*size(zgrid,2)
    if zgrid(i) == min(min(zgrid))
       b1 = [b1 i];
    end
end
b1

b1 = 1×21

21 42 63 84 105 126 147 168 189 210 231 252 273 294 315 336 357 378 399 420 441