I use a size 4 for most of my fishing, with big or small baits, bottom baits or snowman.
I keep saying it, but look at where you are hooking (or losing) fish.
If the fish is hooked in the middle of the bottom lip, then the hair length is right.
If hooked at the back of the mouth, or scraped down the back to the middle, or even the cheek, then usually the hair is too long.
If you have hooked the fish right at the front of the lip, or are losing fish then the hair is too short.
The hair length gap is the distance between the hook and the bait, so it is that that needs changing.
All that gets confused and screwed however, as your background baiting can change how long a hair you need; usually the more groundbait, pellets or particles you use as your baited area may see you need to shorten the hair.
Next thing is pop-ups I tend to fish tight to the hook as the mechanics may be different