Letting your table grow dynamically might not an efficient coding approach. It would be better if you save the tables in a cell array and then attach them together after the loops are completed. For example
regions = cell(1,n)
for i = 1:n
regions{i} = regionprops('table', ..)
end
all_regions = vertcat(regions{:});
Also, this approach does not require you to manually initialze the table.