Sorry, I've been typing about this issue so much that I'm getting sloppy. The opening you have looks suitable for your engine for maximum speed.

The sizing equal to the cylinder is a starting point for general use models. Everyone interested in maximum speed expects to work down from there, and most have some experience and rely on that to figure their first opening size. And often expect to work that down.

As for the average model that's not a pure speed model:

ARFs often come with pre-cut openings. They're often larger than would be required for the engine you would select. So you close off enough of the openings until the right amount of opening remains. You do that in such a way that the opening that remains is in front of your engine.

BTW, ARFs also usually come with little or no exhaust openings directly behind where the engine cylinder winds up.

The Sukhoi31 below came with a solid fake engine plate. It's painted flat black in the picture. There are actually two holes cut into it. There is the large one directly in front of the cylinder. There is a smaller, harder to see one to feed cold air directly at the muffler. What isn't shown are a couple of foam baffles I made to route the air inside toward the opening for the hot air exit.

The picture of the Cap's red cowl shows the two large openings the ARF mfg cut into it. At first, the opening on the right might not look open at all, but it is. I glued a foam baffle just inside the opening and angled it to deflect incoming air toward the crankcase and the muffler. The total openings are way more than the suggested size because the airplane's purpose does not include "fast" or "speed".