Hmmm, day to day riding i've got mountain horse short boots, which are fairly chunky, although other riding i've got long leather boots, which are a lot slimmer. I might go for the 4.75. I can always change if wrong
Test them!! .. When your in the shop wear your riding boots (the ones you wear most often) and place you foot in the iron..
... make sure you have room either side... not so that your feet are 'drowned' with room.. but also be careful that there isn't too little room that your foot could become wedged. xx
We have rather large stirrup irons on pickle as he is on working livery and is mainly ridden my large men! and my dad is 6ft 3 and size 12 I just borrow my sisters spare ones at the moment I must get my own becuase I cant use the HUGE ones!
Or stand on a ruler in your riding boots (go with the widest ones) and measure the ball of your foot where the stirrup sits, the general rule is that you should be able to fit a finger each side of your boot (so add about 1") to the measurement.