I have finally found a collar that fits my girls!!
I have been searching for months now for collars that would fit my girls, all have been waay too big for them as they have quite petite necks. Turns out I didn't need to be looking on any pet websites or searching every pet store for a collar that is around 4/5 inches at it's smallest and finally I've come across some, good 'ol Walmart! Their kitten collars have worked perfectly for my girls! Now I just need to find a good harness that they can't wiggle out of! 

Are you having them wear the collars all the time or just when outside? I would be careful that there isn't anything they can snag on if wearing all of the time. I only have the harness for when they go outside. Mine don't like outside much though and hate the harness even more.

 
Fuzzbutts Rule!

They wear them all the time, there isn't anything they can get it caught on so they're safe and if it does get caught it's a breakaway collar and I've made sure it would be easy to break away if they do get caught. One of them needs it so I can hear the bell and know where she is. They only wear the harness we have for them now when we go out for walks but one of them knows how to slide her way out of it so I'm trying to find new harness for them so I know they'll  be safe when we take them for walks. 

We used collars in the beginning too.
Pirate was a wee lil kit at the time so he grew up wearing it whenever he was out of his cage (we needed the ferret GPS {bell}as new 1st time ferrants) . It was also a means of ferret proofing as we would leave the lead on as well (always supervised so no entanglements) so if he started to get somewhere he shouldn't we could grab the lead and reel him in. We have since given up on collars in the house (although he does have some cool collars with little skull and crossbones on em) and I do miss the happy jingle of a bell on a ferret collar, I think that little bell will always remind me of our lil Pirate boy. 

You will NOT find a collar they can't slip out of, harness maybe, but really if she wants out she will get out, they are so small and flexible they always find a way. We tried with no success to harness Captain and Morgan when we adopted them, different sizes, shapes, makes, of harness' found a way out each time, Pirate was just used to it but he could escape if he really wanted to. 

If you are just using them for walks you should be OK, most ferret escapes from the harness come from the Back out method as opposed to pushing out the front/neck. So when walking if you feel some resistance and fuzzy is trying to back up, its time to scoop up the lil Houdini before the breakout. Just need to be mindful when walking your fuzz outside

good luck

I gave up on collars a long time ago, after my first ferret lost like 5 of them.

THE DOOKS OF HAZARD

I gave up on them to and I have a drawer full of them. They get out of harnesses too so I have a play pen that has a floor and a top and that is what they go in when outside. They can't get out of it but they can get fresh air it is plenty big but I only put five in it at a time and they can play outside in the shade and get some fresh air I never take them for walks they get plenty of walking in my home and I don't have to worry about losing them.

I don't mind if they can slip out of them I just like to hear the jingle to know where they are. They've taken to them quite well and they haven't tried taking them off. The harnesses are becoming the problem as Peppi knows how to slip out of them where she's so tiny. Sophie doesn't really care and will keep it on and stay close to me. There was never really a problem with the harnesses until last week where Peppi thought no I want to go this way, slipped out and I had to run after her where she made a break for it, got underneath the park fencing and almost got to the road. I'm no longer taking them outside until I can find a decent harness and a park where the fencing is tight to the ground so she can't run off. 

I don't use collars for multiple reasons. One is I wouldn't like to have a bell around my neck 24/7, and especially in ferrets who rely greatly on hearing as a sense. Two is collars can chaff their neck and wear the fur away. Now another big reason most people don't know about is coyotes. Coyotes have been trained that the sound of a bell means there is a meal nearby. Like someones cat. An escaped ferret with a bell is quite literally a dinner bell to them.