Can I change the loop size inside the loop?

For loop conditions are set upon entering the loop and the only form of modification that can be made is to 'break' out of the loop early. If you need a indeterminate loop duration use a while loop where the loop condition is checked upon every iteration. From the for loop docs:

Avoid assigning a value to the index variable within the loop statements. The for statement overrides any changes made to index within the loop

Although this only explicitly mentions the loop index variable, it also extends to the end counter:

s=5;

for iter=1:s
    iter
    s=3;
end

>> test

iter =

     1

iter =

     2

iter =

     3

iter =

     4

iter =

     5

As can be see, the attempt to assign 3 to the loop end condition has been ignored