how one can decide number of element or element size?

while finding the resultant of the following component i came across some issues..that case 1) the component with 20 elements and 50 as element size gives 72.5N/mm2 as stress..and case 2) the component with 10 elements and 100 as element size gives 68.2 N/mm2 as stress.. and hand calculation gives 72.92 as shown in fig..my question is how one can decide number of element or element size..