I've flown two legs with Comair this week. Johannesburg to Port Elizabeth and Port Elizabeth to Cape Town.

From Johannesburg, I had a mobile boarding pass, so didn't have to visit the check-in desk. At the gate, there was no gauge and no-one was really interested in any sort of boarding process, we all lined up against the wall with no priority boarding. However looking at carry on, it seemed passengers were certainly keeping at least to the dimensions.

From Port Elizabeth, I was unable to check in online (equipment change and downgrade). At the desk they asked if I had anything to check in, to which I said no, hand luggage only please. This was fine and I received my boarding pass. I had a small laptop bag and small carryon (which was roughly 7Kg in weight. At the gate, there was no gauge, but priority boarding was enforced even through there was no club/business due to the equipment change (A 737-800 all economy). Again on the flight, there was very little carry on and it was all small bags. Nothing like any UK flight where the bins are rammed with carry on.

No means a scientific test, but if you are within dimensions and it doesn't look too heavy. Fingers crossed for my final domestic leg out of Cape Town.