I have one which I used for my Syrian hamsters. It was great in terms of size but they couldn’t climb any bars and it was very poorly ventilated.the lid is very flimsy, dogs and cats would get it off easily. My naughty Syrian used to climb on top of the water bottle and chew the lid to get out! In the summer I had to ensure I had a fan near her due to the ventilation. . It’s definitely not ideal for Guineas, even the larger one. My Syrians was the medium sized.