* Check your owner's manual: This is the most reliable source. The manual will specify the recommended wheel size and tire size for your specific vehicle.
* Look at the tire sidewall: The tire size is printed on the sidewall of your tire. This information includes the wheel diameter. For example, a tire size like "265/70R17" indicates a 17-inch wheel.
* Check the wheel itself: The wheel diameter is usually cast into the wheel itself.
Without knowing the year and model of your Chevy Avalanche, it's impossible to give you a definitive answer. Wheels ranged from 16 inches to 20 inches in diameter across different model years.