I've posted the dimensions at least three times recently 
It should have:
Fronts 296x28 vented.
Rear 286x20 vented.
Sounds like someone fitted the EARLY four pot stuff if they're smaller.
Also, I told you so re the spare... And yes, the tyres are underated.
My MV6 came with none vented discs on the rear with a caliper designed for a wider disc. It was fine until the pad wore out enough for the piston to pop out. The failure mode had some warning. The pad started to get stuck between the disc and the caliper causing some interesting grinding noises. Because I'm a chartered manufacturing engineer with skills in risk assessment, safety, reliability, I obviously ignored this until the brakes went very spongy.
And smoke was pouring from the rear wheel, where the piston had started to release brake fluid as it had travelled past the seal in the bore.
Just before I was about to drive over the Cat and fiddle pass.
I did debate NOT buying new wider discs, but just putting one of the old pads behind the newer pad and saving myself the cost of 2 discs. But that day I felt especially flush and did the job correctly. Getting hold of new linger pins was a hassle. Took me 3 weeks to track them down.
For the avoidance of doubt, I'm probably the laziest Engineer I know.
