I agree- size does matter! A larger bead definitely is easier to see, but I prefer to be able to plant it into the middle of even the small targets. Depending on the light, the smaller bead either looks like a dark post, or round bright bead - again, depending on the light source. For a hunting sight, an 1/8" bead works well and catches light most of the time.
Blades, for me, become very much thinner in appearance as well and wander around inside the wide V of the rear sight's notch - yet a change of light and they are full width. I find this is something I have to live with and manage to shoot reasonably well with them, but the bead and wide V are my favourites. I have the notch and blade on 2 rifles, 2 with wide V's and beads. I do prefer the bead as hazy or fuzzy doesn't matter as the centre of the haze or fuzz is the point of impact. With a blade, if hazy or fuzzy, it 's hard to tell where the top of it is, which is the point of impact, while the bead doesn't give this problem. different barrel lengths give different sight pictures, that's for sure.
If someone wanted to get the perfect sight picture, he'd make a device as Jerry described, then pick a rifle style that fitted those sight distance measurements. One might find he needed a longrfile's length when he actually wanted to build a short 1/2 stocked rifle.