Yes it is big enough, and yes it is about 3ft, I think.
Quite simply, you're wrong.
http://www.guineapigcages.com/ sorry but there it is in black and white. More importantly you are contravening the RSPCA recommended requirements for TWO, which is 4' x 2' - leaving you quite open to being reported to them by a concerned neighbour
I wanted them outside because I don't like guinea-pigs living inside for one thing, every time I walked into my bedroom, every time I'd move by their cage, if I'd simply roll over in bed to get comfy, they would run riot around their cage. I don't want to keep scaring them, basically.
They do get used to these things, as will you, my guineas living outside used to freak every time I opened the back door - two weeks later and they don't. It's all about getting used to their environment. They will get worse rather than better being outside as chances are you will spend less time with them. I find it surprising they weren't used to you being inside as surely you spend a good few more hours a day in your room, awake, than in the garden, especially now it is getting chillier'
August/September is the perfect time to get them outside. These past few months, they'd go outside in their runs for a good 5 hours at least, most days a week, and I leave them out until it gets dark some evenings, so they are very used to the change in temperature from day to night as well as how it's been changing during the year. Besides, these past few months have been all over the place, weather wise. June and July were particularly cold this year.
Speaking of rabbits, mine has never spent a winter inside in the 9 years we've had him, this'll be his 10th year living outside.
Once it gets dark, it can drop a further 3, 4, even 5 degrees - you go outside at 8pm when it is now dark out (boo

goodbye summer) and then at midnight. You might not need a jumper at 8pm, you probably will a bit later. I know, I do it every night for my own guineas.
And, speaking of rabbits, they are NOT the same - rabbits have fur, guineas have hair. Rabbits fur adjusts to the temperature like cats and dogs, guineas doesn't and is not as thick, which you can tell just by stroking the two. Even my hugely floofy Swiss pigs may be hairer, but the density of the hair is still not that of a rabbit's fur
I think you have all forgotten how hardy guinea-pigs actually are. They may be a domestic pet, but they don't need wrapping up in cotton wool all their lives. I'm not an inexperienced or irresponsible owner here, I think I understand pigs a lot better than most of you and that's not meant to sound arrogant. Love the fact I'm being patronised here lol
For someone who seems to think they know more than other members who have done the outside thing properly for a good few years, you might want to remember yes, they are hardy, based on their natural climate of South America - fair bit hotter than the UK so I am told. No they don't need wrapping up in cotton wool but insulation with polystyrene sheets, some snugglesafe pads, then an insulating cover, then a rain proof cover, then extra hay and bedding, then some fleecey cosies, would do fairly well instead. Those are my winter plans and these and only these to this length would satisfy a rescue letting me adopt from them, and darn right. All within a properly sized cage that will allow them to still run about when they can't get out every day, but then we have already covered that one.