Crutchfield is correct. They are a 5" speaker... and no, a 5 1/4 will not fit. A 4 will, but leaves a gap, so a plate needs to be fabricated to close the openings.

You can go to a junk yard or www.hondaautomotiveparts.com and get the LX speaker mounts... they accept 6 1/2's.

Edit: Here is a link to honda automotive parts, this shows the speaker brackets used in your car... the top one (#16, 18) is the box/speaker assembly in your EX, #1 is the bracket for an LX, these are what you'd want to be able to put 6 1/2" speakers in the doors.