You've opened a can of worms asking about "acceptable" and "supposed to".
I think most folks here will say that the wide belt looks better, and I agree with them. If you have a leather strap on your sporran, a narrow belt might "blend in" with it, and not get noticed.
The belt is not to hold up the kilt, anyway, and is not a required accoutrement.
I personally prefer to wear just a sporran instead of a belt, but last weekend at Loch Norman I had a new belt to show off, so I wore both.
I would suggest to you that if you don't have a wide belt, then go without a belt, but if you want to wear a narrow belt, then wear it.

Welcome aboard, by the way, form western North Carolina, the greatest place on Earth.