They are just that -- guidelines, not strict rules. Just go wherever you feel most comfortable. Everyone is pretty easy-going about it. If your are unsure, just ask the others in the small dog area first. My dog is also on the borderline between the two weight groups, and I've never had anyone who wasn't cool with it.
Signs are just a guideline, you know your dog better than anyone to know if it can behave on the small side. Some owners freak out over little things and judge dogs by their size without seeing how they behave first. I bring my mini Aussie only to the small side as the big side has lots of XL aggressive dogs.
Should be fine in the small park if not aggressive. Or use the private yard Signs say small dogs are generally 30# or less.
Check the signs
My pug only knows the big dog side! Those little dogs can have attitudes...haha!
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