In Old Money (for I am ancient ) 60x160cm is about 2'x5'2"
That would be minimum for 3 guineas.
But I would say (IMO) that is way too small. Two at a pinch.
My guineas had a indoor cage 4'x2' that was only used for sleeping (up to 2 hogs) or quarentining a new pair or a going steril boar .
My Pighouse was about 6'6" x 6'6" and I had 5 at one stage . We had to divide the space with C&C barriers and each group had a big sleeping areas ( rabbit hutch on the floor with a door cut out and one of those plastic garden cushion trunks with 2 doors cut out) . They were open to the floor space for each group.
They do use the space , just for pootling.
I liked the playhouse . DD and I could go in, nearly stand up
There was a light, a fan, I had a little heater . Two of the windows could be removed , the space was re-inforced with bars/chickenwire and mesh- to keep out flies, foxes ,insects .
The heated dark room may be fine in colder weather if its completely dry . (They cannot cope with damp ) . They'll burrow in the hay anyway . Can you put some light in there? Even a rechargeable low level one ?
Squirrels are destructive with wires and chew wood etc. We had one in our garden and it is fearless
.
If they do chew then rats and mice can get in. They'll steal the pigs food . I don't know if a rat would go for a guinea , but piggies are extremely timid .
Be aware , there's a bacteria in wild mouse pee that can affect pigs . I had a healthy big boar die very suddenly ( he had vet treatment , subcutaneous infusions , hand feeding , the lot ) He had a sudden attack of the runs (liquid). When I contacted the Rescue (to tell them that he'd gone ) they had a sow with the same symptoms .
My two sows didn't get anything and I dont think the Rescues other pigs did. Not sure what caused it , but my digging brought the mouse pee .
Can you enlarge to space to give you enough for 3 pigs?
Three sows are lovely ( but they can be territorial)
Two boars ( don't keep a boar trio , it is hard work)
Or a neutered boar (only one in a group) and a couple of sows .
Boars are lovely . Gentle , cuddly , soppy .
We've had a couple of soppy sows but mainly they are looking for the next World Domination Project .
We've had two neutered and two entire boars (at different times) all lovely . 
You'll need good dry storage for hay and newspapers too .
Good Luck 