Although it is true that cucumbers are not bad for dogs, it is highly recommended that you avoid giving them to your pet because they can cause stomach aches. If you do give your dog cucumbers, you should make sure to do so in moderation and also cut them up into appropriately-sized pieces.
If you have a larger dog, you will not have to worry about the size of the pieces so much as if you have a smaller dog. There is nothing toxic whatsoever about cucumbers which will make them poisonous or dangerous for your pet, but certain dogs can have negative reactions to them.