Well done - snaring can have a sharp learning curve with many thrills and spills along the way to being as good as some of the pros on here. You will have frustrating days too but look on them as a lesson on the path to improving your skills to the level that you want.
The bigger the better IMO - make sure you use good quality galvanised steel wiring. Might be a bit more expensive but will give you more satisfaction, durablilty and longer life.
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I assume the OP is talking about bait boxes for rodents around farm buildings - at least I hope thats what he means for the sake of his own business reputation, farmers sheepdogs and being on the right side of the law etc!!
A sharp drop in bunny numbers would suggest an outbreak of myxie - if you can't lamp the foxes, I would suggest getting a good fox trap. Which is very handy to have anyways.