To expand on what others have said, the propeller is your airplane's transmission. By selecting a propeller, it's very much like selecting a gear on a stick shift car.

Each propeller is described by 2 numbers. In your case, it was 11x8. That means that the prop is 11" in diameter, and in a perfect world, each revolution will carry the prop forwards 8 inches. Increasing either number increases the load the engine sees (dragging the RPM down), and decreasing either number decreases the load the engine sees (allowing the RPMs' to climb).

A propeller with a large diameter and a small pitch will act like 1st gear in your car. It'll accelerate well, go uphill well, but top speed will be quite low. Conversely, one with a small diameter and a large pitch will act like 5th gear. Acceleration for take off will be very slow (if it goes at all), but once in the air, the speed will be quite impressive. It's climb performance will also suffer.

When we choose a prop for a plane, it's seldom done just by the numbers, tho they're important. An engine manufacturer will give a recommended prop size for starting out with an engine. For a .61, something like a 12x6 or 11x8 might be suggested, but that's by no means the only size we can run. The manufacturer is simply giving us an idea of the sort of load the engine will require. We can move around these numbers a bit to achieve what we want from our plane.

If we want to fly 3d aerobatics, then we want excellent thrust and vertical performance, but high speed is not desirable. We might find the 12x6 to provide too much speed and inadequate vertical. So, to get more thrust, we'd move up to a 13" propeller. In order to keep the engine's speed up, we'll have to lower the pitch. Each inch of diameter is worth 1 to 2 inches of pitch, so a 13x4 or 13x5 might be good things to try.

If it was a speed plane we were setting up, we may find that while our vertical and take-off perfomance is great, our top speed may be disappointing with the 12x6. So, we might try an 11x8, or a 10x9 or something like that.

When swapping props about, its often a good idea to use a tach to verify that the engine's turning RPMs within a reasonable range. For most middle size 2c glow engines (say .40 - .91), between 10,000 and 13,000 is a good rule of thumb. The larger ones will be on the slower end of that range, and the smaller ones tend to want to turn faster.

That 9x6 may well have been run, but I sincerely doubt it was on a .61. If the .61 had started at all (the small prop is not much of a flywheel, and so the engine may have refused to run), it would have grossly overspeeded, and probably come apart in short order. A 9x6 is appropriate for something in the .25-.35 range.

Hope this helps.

J