If you want to fly a 5′ x 8′ American flag there are a few things you must consider. First, what size pole would accommodate a flag that is 40 sq ft.?
This is a 25′ flagpole with 5×8 American flag flying. Use the guideline that the length of the flag should be from 1/4th to 1/3rd the height of the flagpole. (See flag size chart.)  This would make the appropriate flagpole either a 25′ or a 30′ flagpole. OK, now if you are considering flying two flags on your 25′ flagpole you need to take in consideration that 40 sq ft is what is recommended. So if you want a second flag you should use a 4′ x 6′ and a 3′ x 5′  (24 sq ft + 15 sq ft = 39 sq ft) flag combination. This scenario removes the 5′ x 8′ American flag from the 25′ flagpole. So, if you want to fly two flags and retain the 5′ x 8′ American flag size you would need a 30′ flagpole.  This pole allows for a 5′ x 8′ or a 6′ x 10′ American flag, or 60 sq ft.  This would then allow for a 5′ x 8′ American flag and a 4′ x 6′ state flag, 40 sq ft X 24 sq ft = 64 sq ft.  One more item to consider is the strength of you flagpole.  If the pole is a new one piece aluminum flagpole or a one piece fiberglass flagpole then no worries, but if the pole is old, thin, or decaying you should consider flying smaller flags.  With this knowledge choose how many flags you will be flying and your flagpole size, then get out and hoist some flags. 

Common size range chart 

15’          2’x3′ – 3’x5’
20’          3’x5 ’- 4’x6’
25’          4’x6’ –  5’x8’
30’          5’x8’ –  6’x10’
35’          6’x10’ – 8’x12’
40’          6’x10’ – 10’x15’
45’          8’x12’ – 10’x15’
50’          10’x15’ -12’x18’
60’          10’x15’- 15’x25’
70’          12’x18’ – 20’x 30’
80’          15’x25’-  20’x38’
90’          15’x25’ -20’x38’
100’        15’x25’ -30’ X 60’