I'm not 100% convinced that the answer is so simple, potentially. Did you purchase the goods outright, or on finance/HP? Under Sale of Goods Act, s14(2B)(c) requires the goods to be 'free from minor defects'.
Under the Sale of Goods Act, there is a right of repair, but a demand for a repair must be reasonable in relation to all alternatives. Do you feel that a repair is an unreasonable demand when considered against the cost of the sofa? The seller may offer you a repair, free of charge- I'm unsure as to whether a demand for a refund is right here, because I can't see the goods! As you can imagine, if the sofa could be repaired perfectly for £50 and the refund would be £500, it could be a tricky argument.
The seller is responsible for the safe transit of goods, so as long as you had a reasonable time to inspect the goods before accepting them, the seller is prima facie liable for the damage (or at least the hauliers are liable- the point is, you're not.) You said that you made the company aware of the defect upon delivery, so this shouldn't pose a large issue.
The measurements are more intricate. 3 inches may not seem like much, but I guess that if you can't fit the chairs into your house, then it becomes a big issue. To begin with, I'd check over all the paperwork you signed just to be sure they didn't alter the sizes on the invoices. If they did, and you signed, you're stuck with them. That's called the 'last shot doctrine'.
The section on 'satisfactory quality' of goods is one of strict liability. This means that the seller is liable to you and not the manufacturer, so don't be fobbed off my being referred back to the manufacturer!